A trilogy is a set of three works of art, usually literature, film, or video games, that are connected and can be seen either as a single work or three individual works.

Most trilogies are works of fiction involving the same characters or setting, such as The Deptford Trilogy of novels by Robertson Davies or The Godfather films of Francis Ford Coppola. Others are connected only by theme: for example, each film of Krzysztof Kieślowski's Three Colors trilogy explores one of the political ideals of the French Republic (liberty, equality, fraternity) and each novel in Paul Auster's The New York Trilogy uses formats from detective fiction to explore existential questions. Trilogies can also be connected in less obvious ways, such as "The Nova Trilogy" of novels by William S. Burroughs, each written using Brion Gysin's cut-up technique.

Trilogies — and series in general — are common in science fiction and fantasy because of the artistic importance of complex ideas and the commercial importance of brand names.

Occasionally, the term is applied to music, such as the Berlin Trilogy of David Bowie, linked together by their musical sound and lyrical themes, and the fact that part of them was recorded in Berlin, Germany.

A trilogy is different than a triptych, which is three related or connected paintings that are created at one time and designed to be viewed as a single work.

Ancient trilogies

Trilogies date back to ancient times.

In the Dionysia festivals of ancient Greece, for example, trilogies of plays were performed followed by a fourth satyr play. The Oresteia is the only surviving trilogy of these ancient Greek plays, originally performed at the festival in Athens in 458 BC. The three Theban plays, or Oedipus cycle, by Sophocles, originating in 5th century BC, is not a true example of a trilogy because the plays were written at separate times and with different themes/purposes.

In ancient India, an example of an early trilogy includes the epic Mahabharata, which originally consisted of three portions. Vyasa's original core portion of the epic was the Jaya. Vaisampayana's Bharata expanded on the story, with Vyasa's Jaya embedded within it. Ugrasrava eventually composed the Mahabharata, with both Vyasa's Jaya and Vaisampayana's Bharata embedded within the epic.

Adding works to an existing trilogy

Creators of trilogies may later add more works. In such a case, the original three works may or may not keep the title "trilogy."

Richard Wagner's epic series of operas, Der Ring des Nibelungen, is sometimes referred to as a trilogy even though it consists of four works: Das Rheingold, Die Walküre, Siegfried, and Götterdämmerung. The first work, Das Rheingold, is more correctly considered a prelude despite the fact that it is longer than most operas. After this prelude, which sets up the story, the trilogy begins with Die Walküre. Performances of The Ring are usually billed as three nights plus a prelude.