Fair use allows individuals to use of copyrighted materials in specific circumstances:

• to criticize or write commentary on a copyrighted work
• for news reporting
• for teaching, scholarship, or research

These fair use provisions support the principle of promoting "progress of science and useful arts" (quoted text from the U.S. Constitution, Article 8, Section 1, Clause 8 which lays out the principles of copyright).

Just like copyright itself, fair use has limitations. Those limits are defined in U.S. Copyright Law 17 U.S. Code § 107 and are often called the Four Factors. When evaluating the use of a copyrighted work, these factors determine if the use is acceptable or infringes on copyright. The Four Factors are:

1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
2. the nature of the copyrighted work;
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work. (17 U.S. Code § 107)