The Illuminatus! Trilogy is a series of three novels by American writers Robert Shea and Robert Anton Wilson, first published in 1975.1 The trilogy is a satirical, postmodern, science fiction-influenced adventure story; a drug-, sex-, and magic-laden trek through a number of conspiracy theories, both historical and imaginary, related to the authors' version of the Illuminati. The narrative often switches between third- and first-person perspectives in a nonlinear narrative. It is thematically dense, covering topics like counterculture, numerology, and Discordianism.

The trilogy comprises three parts which contain five books and appendices: The Eye in the Pyramid (first two books), The Golden Apple (third and part of fourth book), Leviathan (part of fourth and all of fifth book, and the appendices). The parts were first published as three separate volumes starting in September 1975. In 1984 they were published as an omnibus edition and are now more commonly reprinted in the latter form.

In 1986 the trilogy won the Prometheus Hall of Fame Award, designed to honor libertarian science fiction.2

The authors went on to write several works, both fiction and nonfiction, that dealt further with the themes of the trilogy, but they did not write any direct sequels.

Illuminatus! has been adapted for the stage, as an audio book and has influenced several modern writers, artists, musicians, and games-makers. The popularity of the word "fnord" and the 23 enigma can both be attributed to the trilogy.