The Illuminating Gas, French: Étant donnés: 1° la chute d'eau / 2° le gaz d'éclairage) is a 1966 assemblage by Marcel Duchamp.

It was his last major artwork, surprising viewers and critics who had widely believed he had given up art; he was previously pursuing competitive chess which he had been playing for almost 25 years.

The artwork is a tableau, visible only through a pair of peepholes—one for each eye—in a wooden door, of a nude woman lying on her back on a hill with her face hidden, legs spread, holding a gas lamp in the air in one hand against a landscape backdrop.

[1] It is composed of an old wooden door, nails, bricks, brass, aluminium sheet, steel binder clips, velvet, leaves, twigs, a female form made of parchment, hair, glass, plastic clothespins, oil paint, linoleum, an assortment of lights, a landscape composed of hand-painted and photographed elements and an electric motor housed in a cookie tin which rotates a perforated disc.

[2] Duchamp prepared a "Manual of Instructions" in a 4-ring binder explaining and illustrating how to assemble and disassemble the piece.