Giving credit, using only part of a work, or making no money from it does not automatically make the use lawful. Copyright fair use is a limited defense under U.S. law that allows certain unauthorized uses of copyrighted material when the law sees the use as reasonable and socially valuable, such as criticism, commentary, news reporting, teaching, scholarship, or research. It comes from Section 107 of the Copyright Act of 1976.

Courts do not use a bright-line rule. They weigh four factors: the purpose and character of the use, the nature of the copyrighted work, the amount and substantiality taken, and the effect on the market for the original. A use that adds new meaning or message - often called transformative use - may have a stronger argument, but that alone does not end the analysis. Fair use is judged case by case, which is where many people skid off the road.

Practically, fair use often matters after a copyright infringement claim is made, not before. It can protect a reviewer, teacher, or journalist, but it is not a free pass for reposting photos, music, videos, or written work online. In a lawsuit, fair use can determine whether there are damages, an injunction, or no liability at all. Key guidance comes from cases such as Campbell v. Acuff-Rose Music, Inc. (1994) and Andy Warhol Foundation v. Goldsmith (2023).