Hypothesis is a web annotation tool, allowing users to attach sentence-level notes to any site on the web. No support is necessary for the underlying websites - Hypothesis functions as an overlay that works everywhere. Hypothesis is produced by a non-profit foundation with a list of sponsors including the Sloan Foundation, the Knight Foundation, the Mellon Foundation, and numerous others. Through their work with the W3C Web Annotation Working Group, the Hypothesis project seeks to build not just a working web annotation system, but standards for interoperable annotations of digital documents more generally. Their work is guided by a set of community principles and all their software is available under open source licenses on GitHub. The In Action section of the Hypothesis website provides examples of employing Hypothesis for educators, journalists, scientists, publishers, and the general public. To begin creating annotations, users need to register for an account. Google Chrome users can then utilize a Chrome Extension to interact with Hypothesis. For other browsers, Hypothesis provides a bookmarklet for quick access to the Hypothesis website.