research literature databases

Literature Search: Databases and Gray Literature

The literature search.

• A systematic review search includes a search of databases, gray literature, personal communications, and a handsearch of high impact journals in the related field.  See our list of recommended databases and gray literature sources on this page.
• a comprehensive literature search can not be dependent on a single database, nor on bibliographic databases only.
• inclusion of multiple databases helps avoid publication bias (georaphic bias or bias against publication of negative results).
• The Cochrane Collaboration recommends PubMed, Embase and the Cochrane Central Register of Controlled Trials (CENTRAL) at a minimum.     
• NOTE:  The Cochrane Collaboration and the IOM recommend that the literature search be conducted by librarians or persons with extensive literature search experience. Please contact the NIH Librarians for assistance with the literature search component of your systematic review. 

Cochrane Library

A collection of six databases that contain different types of high-quality, independent evidence to inform healthcare decision-making. Search the Cochrane Central Register of Controlled Trials here.

European database of biomedical and pharmacologic literature.

PubMed comprises more than 21 million citations for biomedical literature from MEDLINE, life science journals, and online books.

Largest abstract and citation database of peer-reviewed literature and quality web sources. Contains conference papers.

Web of Science

World's leading citation databases. Covers over 12,000 of the highest impact journals worldwide, including Open Access journals and over 150,000 conference proceedings. Coverage in the sciences, social sciences, arts, and humanities, with coverage to 1900.

Subject Specific Databases

APA PsycINFO

Over 4.5 million abstracts of peer-reviewed literature in the behavioral and social sciences. Includes conference papers, book chapters, psychological tests, scales and measurement tools.

CINAHL Plus

Comprehensive journal index to nursing and allied health literature, includes books, nursing dissertations, conference proceedings, practice standards and book chapters.

Latin American and Caribbean health sciences literature database

Gray Literature

• Gray Literature  is the term for information that falls outside the mainstream of published journal and mongraph literature, not controlled by commercial publishers
• hard to find studies, reports, or dissertations
• conference abstracts or papers
• governmental or private sector research
• clinical trials - ongoing or unpublished
• experts and researchers in the field     
• Library catalogs
• Professional association websites
• Google Scholar  - Search scholarly literature across many disciplines and sources, including theses, books, abstracts and articles.
• Dissertation Abstracts - dissertation and theses database - NIH Library biomedical librarians can access and search for you.
• NTIS  - central resource for government-funded scientific, technical, engineering, and business related information.
• AHRQ  - agency for healthcare research and quality
• Open Grey  - system for information on grey literature in Europe. Open access to 700,000 references to the grey literature.
• World Health Organization  - providing leadership on global health matters, shaping the health research agenda, setting norms and standards, articulating evidence-based policy options, providing technical support to countries and monitoring and assessing health trends.
• New York Academy of Medicine Grey Literature Report  - a bimonthly publication of The New York Academy of Medicine (NYAM) alerting readers to new gray literature publications in health services research and selected public health topics. NOTE: Discontinued as of Jan 2017, but resources are still accessible.
• Gray Source Index
• OpenDOAR - directory of academic repositories
• International Clinical Trials Registery Platform  - from the World Health Organization
• Australian New Zealand Clinical Trials Registry
• Brazilian Clinical Trials Registry
• Chinese Clinical Trial Registry - 
• ClinicalTrials.gov   - U.S.  and international federally and privately supported clinical trials registry and results database
• Clinical Trials Registry  - India
• EU clinical Trials Register
• Japan Primary Registries Network  
• Pan African Clinical Trials Registry

Database Search

What is Database Search?

Harvard Library licenses hundreds of online databases, giving you access to academic and news articles, books, journals, primary sources, streaming media, and much more.

The contents of these databases are only partially included in HOLLIS. To make sure you're really seeing everything, you need to search in multiple places. Use Database Search to identify and connect to the best databases for your topic.

In addition to digital content, you will find specialized search engines used in specific scholarly domains.

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Research Paper Sources

24 Best Databases For Research: A Comprehensive List For Students

From academic journals to professional publications, this list covers the best databases for research. Start your search for valuable information.

Apr 5, 2024

Research is the driving force behind the innovation and progress that defines every field of study. In the digital age, this pursuit has been significantly diversified, with multiple avenues available for exploration. The most reliable sources of information are databases designed to deliver accurate and updated information on a wide range of subjects. In this blog, we delve into the world of database research and bring to light the best databases for research, delving into the difference between primary vs secondary sources .

Table of Contents

How to evaluate a research database, benefits of using academic and professional databases for research, how to find peer-reviewed articles, common research mistakes to avoid, supercharge your researching ability with otio — try otio for free today.

Research databases are like treasure troves, offering a wealth of scholarly materials such as journal articles, conference papers, books, and more. Each database is meticulously curated and organized, making them invaluable tools for researchers. These databases are segmented to facilitate research across specific disciplines by focusing on distinct fields or fostering research that transcends multiple domains. 

They are akin to libraries designed to cater to the nuanced demands of the academic community, providing a structured digital environment where researchers can delve into a curated assortment of resources that are both focused and wide-ranging.

Criteria for Evaluating Research Databases

Coverage and scope.

When determining the credibility and reliability of research databases, one must first assess their coverage and scope. A database's value is contingent on its ability to index a broad array of scholarly literature—from peer-reviewed journals and conference proceedings to academic books and other relevant sources. The broader the scope and coverage, the more likely researchers are to find a comprehensive array of material that aligns with their field of interest.

Content Quality

Another essential factor in evaluating a research database is the quality of its content. Key considerations include the reputation of the journals and publishers contained in the database, the stringency of the peer-review process, and the credibility of the authors and institutions involved. Quality control is essential when ensuring that the content within a research database is trustworthy and academically sound.

Search Features and Functionality

The search features and functionality of a database are vital for researchers who rely on it to find relevant information. The capacity to refine search queries using filters for publication date, document type, or subject category is crucial. The user interface, accessibility of search results, and general ease of navigation are pivotal features that contribute to the overall usability and effectiveness of a research database.

Peer Review

A significant mark of a reputable research database is its inclusion of peer-reviewed journals and publications. Articles that have undergone rigorous scrutiny by domain experts are inherently more trustworthy and credible. Prioritizing peer-reviewed content ensures that researchers have access to information that is both authoritative and accurate, ultimately enhancing the quality of their research endeavors.

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Reliable Resources

When conducting research, it is crucial to use reliable resources that provide accurate and trustworthy information. Research databases are excellent sources of information because they contain articles, studies, and other resources written by professionals within specific fields. These sources undergo a rigorous editorial process and are peer-reviewed, ensuring their accuracy and reliability. 

This level of quality is far superior to information found through regular search engines, where results can be manipulated and sources can lack credibility. By utilizing research databases, researchers can access materials that are not only verified but also written by experts in the field. This ensures that the information is current, accurate, and trustworthy, helping researchers find the support they need for their work.

One of the key benefits of using research databases is the availability of accurate citations. Research databases include citations for any information sourced from other works, providing a clear trail of references that can be useful for further research. These sources usually come with a citation that can be directly used to reference the material in a research paper or presentation. 

This feature helps save time by providing an accurate citation for the source used in research work and ensures that authors give credit to the original work. By including citations, research databases facilitate the proper acknowledgment of sources and give authors confidence in the accuracy and authenticity of their work.

Accessibility

Research databases offer easy access to a range of peer-reviewed, scholarly sources, making it simple for professionals to find reliable materials for their research. These databases often contain a wide variety of content, including academic journals, articles, reports, and more, allowing researchers to explore a wealth of resources relevant to their field of study. 

Peer-reviewed sources are valued in academic and professional circles because they undergo a rigorous evaluation process by experts in the same field. This evaluation ensures the accuracy, credibility, and quality of the content, making it a valuable resource for researchers. Access to these resources is essential for researchers to stay informed about the latest developments, trends, and findings within their area of interest.

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Knowledge workers, researchers, and students today suffer from content overload and are left to deal with it using fragmented, complex, and manual tooling. Too many of them settle for stitching together complicated bookmarking, read-it-later, and note-taking apps to get through their workflows. Now that anyone can create content with the click of a button - this problem is only going to get worse. solves this problem by providing one AI-native workspace for researchers. It helps them:

A wide range of data sources, from bookmarks, tweets, and extensive books to YouTube videos.

Extract key takeaways

With detailed AI-generated notes and source-grounded Q&A chat.

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Along with this, Otio also helps you write research papers/essays faster. Here are our top features that are loved by researchers: AI-generated notes on all bookmarks (YouTube videos, PDFs, articles, etc.), Otio enables you to chat with individual links or entire knowledge bases, just like you chat with ChatGPT, as well as AI-assisted writing.

PubMed is a free search engine operated by the National Institutes of Health (NIH), focusing primarily on biomedical literature. It includes millions of citations and abstracts from biomedical literature, comprising life science journals, online books, and articles.

JSTOR is a digital library that offers access to thousands of academic journals, books, and primary sources across various disciplines, such as humanities, social sciences, and sciences. It is valuable for historical research due to its full-text access to older journal issues.

Scopus is a comprehensive abstract and citation database that covers a wide range of disciplines. It includes peer-reviewed literature like journals, conference proceedings, and patents. Scopus provides advanced search and analysis tools for researchers.

4. Web of Science

Developed by Clarivate Analytics, Web of Science is a multidisciplinary citation database covering scholarly literature from various fields. It indexes high-impact journals and offers citation metrics to assess research impact and track scholarly trends.

5. Google Scholar

Google Scholar is a freely accessible web search engine that indexes scholarly literature across different disciplines. It includes articles, theses, books, and conference papers sourced from academic publishers, professional societies, universities, and more.

6. IEEE Xplore

IEEE Xplore is a digital library that offers access to research literature in engineering, technology, and related fields. It features journals, conference proceedings, standards, and eBooks published by the Institute of Electrical and Electronics Engineers (IEEE) and its partners.

7. PsycINFO

PsycINFO is a bibliographic database maintained by the American Psychological Association (APA). It covers psychology and related disciplines, including psychiatry, sociology, education, and anthropology. PsycINFO indexes journal articles, books, chapters, and dissertations.

8. ScienceDirect

ScienceDirect is an online platform operated by Elsevier, providing access to a vast collection of scientific and technical research literature. It hosts journals, books, and reference works across various fields like physical sciences, life sciences, health sciences, and social sciences.

9. SpringerLink

SpringerLink is a digital platform offering access to scientific, technical, and medical content published by Springer Nature. It includes journals, books, conference proceedings, and reference works in numerous disciplines, including natural sciences, computer science, and medicine.

10. BioMed Central

BioMed Central (BMC) is a publisher of open-access, peer-reviewed journals in the fields of biology, medicine, and biomedicine. It offers free access to research articles, reviews, and other content, promoting transparency and accessibility in scientific publishing.

11. PubMed Central

PubMed Central (PMC) is a free digital repository maintained by the National Library of Medicine (NLM). It archives full-text scholarly articles from biomedical and life science journals, ensuring long-term preservation and accessibility to research literature.

Education Resources Information Center (ERIC) is an online digital library of education research and information. It provides access to articles, research reports, conference papers, and other resources related to various aspects of education, including teaching, learning, and policy.

13. ProQuest

ProQuest is a multidisciplinary research database that offers access to scholarly journals, newspapers, dissertations, and other publications. It covers a wide range of subjects, including arts, business, health, science, and social sciences, providing valuable resources for researchers.

14. EconLit

EconLit is a database maintained by the American Economic Association, focusing on economics literature. It includes journal articles, books, working papers, and more, covering topics such as economic theory, labor economics, international economics, and economic development.

Cumulative Index to Nursing and Allied Health Literature (CINAHL) is a comprehensive database of nursing and allied health literature. It includes journal articles, books, dissertations, standards, and more, helping healthcare professionals access relevant research in their fields.

16. PsycARTICLES

PsycARTICLES is a database offering full-text articles from journals published by the APA and allied organizations. It covers a wide range of psychology topics, providing access to research articles, reviews, and other content for professionals and researchers in the field.

17. EconBiz

EconBiz is a research tool for economics and business studies, offering access to academic literature, journals, working papers, and other resources in the field. It covers topics such as economic policy, finance, management, and more, providing valuable information for researchers.

18. IEEE Computer Society Digital Library

The IEEE Computer Society Digital Library is a platform providing access to research literature related to computer science and technology. It includes journals, conference proceedings, and standards published by the IEEE Computer Society, offering valuable resources for researchers in the field.

19. IEEE Transactions on Neural Networks and Learning Systems

IEEE Transactions on Neural Networks and Learning Systems is a journal focusing on the emerging areas of neural networks and learning systems. It covers research articles, reviews, and applications related to neural networks, deep learning, machine learning, and other related topics.

20. IEEE Transactions on Pattern Analysis and Machine Intelligence

IEEE Transactions on Pattern Analysis and Machine Intelligence is a journal that publishes research articles, reviews, and applications related to pattern analysis, machine learning, computer vision, and related topics. It offers valuable resources for researchers working in these areas.

21. IEEE Transactions on Control Systems Technology

IEEE Transactions on Control Systems Technology is a journal focusing on research articles, reviews, and applications related to control systems technology. It covers topics such as adaptive control, process control, robotics, and more, providing valuable resources for researchers in the field.

22. Agricola

Agricola is a database maintained by the National Agricultural Library, offering access to agricultural literature and resources. It includes journal articles, books, theses, patents, and more, covering topics such as agriculture, forestry, food, and nutrition, providing valuable information for researchers in the field.

23. Business Source Complete

Business Source Complete is a comprehensive business research database offering access to scholarly journals, trade publications, industry reports, and more. It covers various business disciplines, including marketing, management, finance, and economics, providing valuable resources for researchers and professionals in the field.

24. Sociological Abstracts

Sociological Abstracts is a database providing access to literature in the field of sociology and related disciplines. It includes journal articles, books, conference papers, and other resources, covering topics such as social behavior , culture, social structure, and more, offering valuable information for researchers in the field. Let Otio be your AI research and writing partner — try Otio for free today!

• How To Tell If An Article Is Peer Reviewed • Reliable Sources For Research • Literature Search • How To Summarize A Research Article • Summarize Research Paper Ai • Using Ai For Research • How To Use Chat Gpt For Research • How To Search For Research Articles

Peer-reviewed articles are the cornerstone of credible research. The internet is flooded with articles, journals, and books, some of which lack credibility . Peer-reviewed articles have undergone scrutiny by industry experts and scholars, making them trustworthy sources for research.

Factors to Consider When Finding Peer-Reviewed Articles 

Check the author.

Peer-reviewed articles are crafted by experts in a specific field, ensuring that the research is credible. This rigid evaluation process requires authors to have industry expertise relevant to their research. When assessing an article, verify the author's credentials to guarantee their expertise.

Choose a Database

Select a reputable academic database or search engine that indexes peer-reviewed literature in your area of interest. This ensures that your research findings are credible and reliable.

Define Your Search Query

Clearly define your research topic or question to guide your search. Identify relevant keywords, phrases, and concepts, and consider utilizing Boolean operators to combine or exclude search terms for more precise results.

Use Advanced Search Features

Leverage advanced search features provided by the database to refine your search results. Filters for publication date, document type, author affiliation, and journal title can help narrow down the most relevant articles for your research.

Select Peer-Reviewed or Scholarly Journals

Look for options to restrict your search results to peer-reviewed or scholarly journals. This filter guarantees that the articles you find have undergone a rigorous review process by industry experts.

Check Journal Information

Ensure that the journal publishing the article is peer-reviewed or scholarly. Review the journal’s editorial board, review process, and publication standards on the publisher’s website to verify its credibility.

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Otio streamlines the research process and accelerates the transition from reading lists to first drafts. Researchers can benefit from Otio’s AI-generated notes on all bookmarks, a chat feature for individual links or entire knowledge bases, and AI-assisted writing, making the research and writing experience seamless and efficient.

Sticking to One Source

When conducting research, it's important to explore various angles and perspectives on your topic. Relying on a single source may limit the depth and breadth of information you can gather and present in your work.

Not Documenting Every Fact

Failing to document each piece of information as you conduct your research can lead to a lot of extra work when it comes time to write your paper or article. Be sure to keep thorough notes and properly attribute all information to its original source.

Using Outdated Materials

Academic disciplines are constantly evolving, and new research may have surfaced since older articles were published. Ensure your research is current by incorporating the latest studies and findings in your work.

Relying Solely on Open Access Journals

While Open Access journals provide a wealth of free articles, don't discount paid content . Exploring a variety of sources, both free and paid, can enrich your research and provide a more comprehensive understanding of your topic.

Digging Too Deep or Not Deep Enough

Finding the right balance in the depth of your research is crucial. Conducting searches that are too broad may yield vague results, while overly specific searches could lead to irrelevant information. Aim for a concise yet thorough exploration of your research question.

Knowledge workers, researchers, and students today suffer from content overload and use fragmented, complex, and manual tools to get through their workflows. Many settle for stitching together complicated bookmarking, read-it-later, and note-taking apps. Now, with anyone able to create content with a click, the problem is only getting worse. Otio is solving this issue by providing an AI-native workspace for researchers. 

Efficient Knowledge Collection and Note-Taking 

This platform helps users to collect from a wide range of data sources, including bookmarks, tweets, extensive books, and YouTube videos. Users can extract key takeaways with detailed AI-generated notes and source-grounded Q&A chat. Otio helps create draft outputs using collected sources, assisting users in progressing from reading lists to first drafts more efficiently. 

Streamlining Research and Writing Processes with Otio's AI Assistance

Otio also aids in faster research paper and essay writing. Key features that researchers love include AI-generated notes on all bookmarks (YouTube videos, PDFs, articles, etc.), individual link or entire knowledge base chat like ChatGPT, and AI-assisted writing. 

Otio can be your AI research and writing partner — try it for free today.

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PubMed® comprises more than 38 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full text content from PubMed Central and publisher web sites.

Featured Bookshelf titles

Drugs and Lactation Database (LactMed®)

Clinical and Research Information on Drug-Induced Liver Injury

Literature databases

Books and reports

Ontology used for PubMed indexing

Books, journals and more in the NLM Collections

Scientific and medical abstracts/citations

Full-text journal articles

Gene sequences and annotations used as references for the study of orthologs structure, expression, and evolution

Collected information about gene loci

Functional genomics studies

Gene expression and molecular abundance profiles

Sequence sets from phylogenetic and population studies

Protein sequences, 3-D structures, and tools for the study of functional protein domains and active sites

Conserved protein domains

Protein sequences grouped by identity

Protein sequences

Models representing homologous proteins with a common function

Experimentally-determined biomolecular structures

A tool to find regions of similarity between biological sequences

Search nucleotide sequence databases

Search protein sequence databases

Search protein databases using a translated nucleotide query

Search translated nucleotide databases using a protein query

Find primers specific to your PCR template

Genome sequence assemblies, large-scale functional genomics data, and source biological samples

Genome assembly information

Museum, herbaria, and other biorepository collections

Biological projects providing data to NCBI

Descriptions of biological source materials

Genome sequencing projects by organism

DNA and RNA sequences

High-throughput sequence reads

Taxonomic classification and nomenclature

Heritable DNA variations, associations with human pathologies, and clinical diagnostics and treatments

Privately and publicly funded clinical studies conducted around the world

Human variations of clinical significance

Genotype/phenotype interaction studies

Short genetic variations

Genome structural variation studies

Genetic testing registry

Medical genetics literature and links

Online mendelian inheritance in man

Repository of chemical information, molecular pathways, and tools for bioactivity screening

Bioactivity screening studies

Chemical information with structures, information and links

Molecular pathways with links to genes, proteins and chemicals

Deposited substance and chemical information

Research news

Study reveals a cell-eat-cell world.

From normal vertebrate development to tumor cell cannibalism, cell-in-cell events occur in many different contexts across the tree of life

Researchers Bioengin-Ear Tissue Scaffolds to Human Scale

A new approach to sculpting human-like ears merges 3D printing, xenografts, and tissue engineering.

Do cats experience grief? New research suggests they might

Researchers from Oakland University surveyed hundreds of cat caregivers and found that cats exhibited behaviors associated with grief after a fellow cat or dog in the household died.

Recent blog posts

Discovery of culprit behind scars in heart failure points to possible treatment target.

More than 6 million adults in the U.S. have heart failure, a condition that develops when the heart can’t pump enough blood to meet the demands of the body. While lifestyle changes and treatment can slow heart failure, there’s no cure. One reason is that heart failure and other heart conditions including heart attacks lead heart muscle to become thickened and scarred in a process known as fibrosis. Effective strategies to reverse or stop fibrosis in the heart or other organs after injury, as is needed for recovery, have remained elusive. Now, a new NIH-supported study offers a step forward in better understanding what happens in human heart failure. These findings, reported in Nature, identify a cell type that may be a main culprit in the formation of scar tissue after heart injury. What’s even more encouraging is that the study, which included mouse models, suggests that existing treatments that block communication between the immune system and the scar-forming cells may hold promise for limiting fibrosis to improve heart function.

RefSeq Release 227 is Available!

Check out RefSeq release 227, now available online and from the FTP site. You can access RefSeq data through NCBI Datasets. The release is provided in several directories as a complete dataset and also as divided by logical groupings. What’s included in this release? As of November 4, 2024, this full release incorporates genomic, transcript, and protein data containing: … Continue reading RefSeq Release 227 is Available! →

DiscoverWHR: Centralizing NIH’s Investments in Women’s Health Research

With DiscoverWHR—a new resource from the NIH Office of Research on Women’s Health (ORWH) and NLM—users now have easy access to NIH-funded research on the important topic of women’s health research. Explore studies, grants, clinical trials, and more on topics like menopause, PCOS, and autoimmune diseases.

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The top list of academic search engines

1. Google Scholar

4. science.gov, 5. semantic scholar, 6. baidu scholar, get the most out of academic search engines, frequently asked questions about academic search engines, related articles.

Academic search engines have become the number one resource to turn to in order to find research papers and other scholarly sources. While classic academic databases like Web of Science and Scopus are locked behind paywalls, Google Scholar and others can be accessed free of charge. In order to help you get your research done fast, we have compiled the top list of free academic search engines.

Google Scholar is the clear number one when it comes to academic search engines. It's the power of Google searches applied to research papers and patents. It not only lets you find research papers for all academic disciplines for free but also often provides links to full-text PDF files.

• Coverage: approx. 200 million articles
• Abstracts: only a snippet of the abstract is available
• Related articles: ✔
• References: ✔
• Cited by: ✔
• Links to full text: ✔
• Export formats: APA, MLA, Chicago, Harvard, Vancouver, RIS, BibTeX

BASE is hosted at Bielefeld University in Germany. That is also where its name stems from (Bielefeld Academic Search Engine).

• Coverage: approx. 136 million articles (contains duplicates)
• Abstracts: ✔
• Related articles: ✘
• References: ✘
• Cited by: ✘
• Export formats: RIS, BibTeX

CORE is an academic search engine dedicated to open-access research papers. For each search result, a link to the full-text PDF or full-text web page is provided.

• Coverage: approx. 136 million articles
• Links to full text: ✔ (all articles in CORE are open access)
• Export formats: BibTeX

Science.gov is a fantastic resource as it bundles and offers free access to search results from more than 15 U.S. federal agencies. There is no need anymore to query all those resources separately!

• Coverage: approx. 200 million articles and reports
• Links to full text: ✔ (available for some databases)
• Export formats: APA, MLA, RIS, BibTeX (available for some databases)

Semantic Scholar is the new kid on the block. Its mission is to provide more relevant and impactful search results using AI-powered algorithms that find hidden connections and links between research topics.

• Coverage: approx. 40 million articles
• Export formats: APA, MLA, Chicago, BibTeX

Although Baidu Scholar's interface is in Chinese, its index contains research papers in English as well as Chinese.

• Coverage: no detailed statistics available, approx. 100 million articles
• Abstracts: only snippets of the abstract are available
• Export formats: APA, MLA, RIS, BibTeX

RefSeek searches more than one billion documents from academic and organizational websites. Its clean interface makes it especially easy to use for students and new researchers.

• Coverage: no detailed statistics available, approx. 1 billion documents
• Abstracts: only snippets of the article are available
• Export formats: not available

Consider using a reference manager like Paperpile to save, organize, and cite your references. Paperpile integrates with Google Scholar and many popular databases, so you can save references and PDFs directly to your library using the Paperpile buttons:

Google Scholar is an academic search engine, and it is the clear number one when it comes to academic search engines. It's the power of Google searches applied to research papers and patents. It not only let's you find research papers for all academic disciplines for free, but also often provides links to full text PDF file.

Semantic Scholar is a free, AI-powered research tool for scientific literature developed at the Allen Institute for AI. Sematic Scholar was publicly released in 2015 and uses advances in natural language processing to provide summaries for scholarly papers.

BASE , as its name suggest is an academic search engine. It is hosted at Bielefeld University in Germany and that's where it name stems from (Bielefeld Academic Search Engine).

CORE is an academic search engine dedicated to open access research papers. For each search result a link to the full text PDF or full text web page is provided.

Science.gov is a fantastic resource as it bundles and offers free access to search results from more than 15 U.S. federal agencies. There is no need any more to query all those resources separately!