A derivative work is a new work built on top of something that already exists—a translation, a film adaptation, a remix, a sequel, an annotated edition, or the next version of a program. This article explains what qualifies as a derivative work under 17 U.S.C. § 101, why 17 U.S.C. § 103(b) confines the copyright to the new material the author actually contributed, and why no adaptation can revive or extend the underlying work's term. It walks the U.S. Copyright Office application field by field—showing how to complete the author, claimant, previous-registration, and limitation-of-claim entries so the examiner sees an accurate story about what you own. It maps the circuit split over how much originality a derivative needs, from the distinguishable-variation rule of Alfred Bell to the substantial-variation rule of Batlin and the hybrid test of Schrock, and it explains the "thin copyright" that even a valid derivative often carries. It covers the § 103(a) unlawful-use trap, the unauthorized-versus-unlawful distinction from Keeling v. Hars, the accuracy standard the Supreme Court set in Unicolors v. H&M, and worked examples across translation, music, software, and visual art.