Legal issues with fan fiction arise due to the prospect that a piece of fan fiction may constitute a derivative work, most prominently (but not exclusively) under United States copyright law.
Significant amounts of copyrightable creative works such as motion pictures, television programs, music, and computer gaming works are produced in the United States. In addition, a significant amount of fan fiction is created in the United States. For these reasons, although every nation's law is different and different laws may apply to different works of fan fiction, U.S. law is often centrally relevant when determining the legality of writing and/or sharing fan fiction.
Under U.S. Copyright law, the legality of a given work of fan fiction will depend principally on three legal doctrines: (1) copyrightability of the underlying source work; (2) the derivative work right; and (3) fair use.
To have copyright protection under U.S. law, a work must be an "original [work] of authorship fixed in any tangible medium of expression . . . from which [it] can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device." Such works of authorship include but are not limited to literature, music, plays, pictures and architectural works. Copyright can not be applied to ideas, concepts, facts or other broad principles regardless of whether they are expressed in a tangible medium or otherwise. Copyright goes into effect automatically, even if a work is not published. For works created in 1978 or later, copyright protection persists for the life of the author plus 70 years; in the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from publication, or for 120 years from the year of its creation, whichever expires first.
According to current United States copyright, copyright owners have the exclusive right "to prepare derivative works based upon [their] copyrighted work." A derivative work is any work, including fanfiction based upon one or more preexisting works. In the case where a copyright owner chooses to exercise his/her exclusive right to prepare derivative works against a work of fanfiction, s/he can sue the fanfiction writer for copyright infringement. To prove infringement, an owner must present evidence establishing that the accused has copied protected elements of the original work and the accused work does not constitute fair use. The possible remedies available if infringement can be proven include an order to cease sharing and/or to destroy the work (known as an injunction), or monetary damages. The remedy is dependent on the harm done to the copyright owner, the intent of the infringing person, and the grievousness of the infringement. An example of injunction as remedy was seen in the case of Anderson v. Stallone. There, Sylvester Stallone successfully pursued an action for copyright infringement against Anderson, an author who wrote a proposed script for Rocky IV, by proving that the copyright-protected characters used in the previous Rocky movies were central to the new script. The court enjoined Anderson from pursuing the creation of a movie or other published work based on his script.