DEMO (DEMOnstration Power Plant) is a proposed nuclear fusion power plant that is intended to build upon the ITER experimental nuclear fusion reactor. The objectives of DEMO are usually understood to lie somewhere between those of ITER and a "first of a kind" commercial station. While there is no clear international consensus on exact parameters or scope, the following parameters are often used as a baseline for design studies: DEMO should produce at least 2 gigawatts of fusion power on a continuous basis, and it should produce 25 times as much power required for breakeven. DEMO's design of 2 to 4 gigawatts of thermal output will be on the scale of a modern electric power plant.

Demo may refer to:

• Demo (music), a song recorded for reference rather than release
• Demo (Behind Crimson Eyes), a 2004 recording by the band Behind Crimson Eyes
• Demo (Deafheaven album), a 2010 EP by Deafheaven
• Demo (The Flatliners Album), a 2002 album by the band The Flatliners
• Demo (Miss May I album), a 2008 recording by the metalcore band Miss May I
• "Demo", a 1990 single by Die Krupps
• "Demo" (P-Model song), a 1979 recording by songwriters Susumu Hirasawa and Yasumi Tanaka
• Demo (Skinless), a 1994 recording by the band Skinless
• Demo (Street Dogs EP)
• Demo 2004 (Year of No Light album), a 2004 recording by Year of No Light
• Demo #2, an unreleased recording by Neutral Milk Hotel