The "fair use" defense to copyright infringement is one of the most amorphous legal doctrines.  Originating in the U.S. courts, and adopted by only a few other countries, it allows "fair" use of copyrighted works without authorization from the owner. The fair use doctrine furthers the constitutional purpose of promoting "the progress of science and useful arts," and has been characterized as fundamental to defending freedom of expression under the First Amendment.  But how much and what use is "fair" versus what use will end up being declared copyright infringement often feels like guess work, especially because courts have arrived at different conclusions when faced with the same or very similar fact patterns. This causes some artists, authors and other creators to refrain from borrowing material from an already existing work, which arguably unduly hinders creativity. Recent case law in this area, including from the U.S. Supreme Court, has done little to bring clarity to this already murky area, but some general principles still hold true, which we discuss here.