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This is a tool to find open-access versions of paywalled research papers: it is currently available as an extension for Chrome and Firefox.  It helps you to source free, legal fulltext of research papers as you browse.

When you view a paywalled research article, Unpaywall automatically looks for a copy in their index of over 10 million free, legal fulltext PDFs. If one is found, click the green tab that appears to read the article.


How does Unpaywall find all these fulltext articles?
They harvest content directly from over 50,000 open-access repositories from all over the world: for example open data from PubMed Central, the DOAJ, Crossref (particularly their license info), and DataCite. Additionally there are millions of research papers are available for free on government and university web servers, legally uploaded by the authors themselves, with the express permission of publishers. Unpaywall automatically harvests these freely shared papers from thousands of legal institutional repositories, preprint servers, and publishers, making them all available to you as you read. 

Is Unpaywall legal?
Yes! They harvest content from legal sources including repositories run by universities, governments, and scholarly societies, as well as open content hosted by publishers themselves. They expressley do not harvest from sources of dubious legality like ResearchGate or Sci-Hub.

Access millions of research articles, instantly
Unpaywall users are able to read over 52% of research papers for free without using pirate sites like Sci-Hub. That's because they have built a database of millions of open-access articles harvested from thousands of servers worldwide. When you browse to a paywalled paper, the software checks to see if there is a free copy. If so, you'll see a green tab on the edge of your browser...just click, and read. They expect the success rate will improveover time.

Unpaywall tab can change colors to indicate Green or Gold OA, too. Enable it in settings. You might also enjoy our recent paper analyzing trends in OA and the coverage of Unpaywall.

Visit: https://unpaywall.org/faq