"Fair use," originally a judge-made doctrine, is a statutory defense to copyright infringement. The test for "fair use," articulated in Section 107 of the Copyright Act (17 U.S.C. Section 107), includes four key factors: the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; the nature of the copyrighted work; the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the effect of the use upon the potential market for or value of the copyrighted work.

Although the statutory test sounds straightforward, it can be one of the most nebulous and difficult concepts to apply. Courts and litigants routinely struggle with the analysis, which is inherently fact dependent and must be considered on a case-by-case basis; bright-line rules are to be avoided. See Campbell v. Acuff-Rose Music, 510 U.S. 569, 575-578 (1994). Moreover, all four factors must be considered and weighed together, keeping in mind the ultimate purpose of copyright "to promote the progress of science and useful arts."