U.D.O. is a German heavy metal band founded by lead singer Udo Dirkschneider in 1987. Closely tied to the name U.D.O. is Accept, founded by Udo Dirkschneider and Michael Wagener (later to become a top producer) in Solingen, which was the predecessor of U.D.O.

U.D.O. is an incorrect tag for Udo Mechels.

After the breaking up of Accept in 1987, a large fan community followed Udo Dirkschneider and Stefan Kaufmann to U.D.O.

Starting with the first albums Animal House in 1988 and Mean Machine in 1989 U.D.O. proved to be pure heavy metal without any Trend-Sound influences. Both albums continued where Accept had left of in 1986 with Russian Roulette. Animal House was played in by Peter Szigeti (guitars), Frank Rittel from Warlock (bass), Mathias Dieth from Sinner (guitars),Thomas Franke (drums) and of course Udo Dirkschneider (vocals)

When Mean Machine was released in 1989 U.D.O. had a totally different look. From the line-up which had played on Animal House, remained besides Udo only Mathias Dieth (guitar). New were Andy Susemihl (guitar), Thomas Smuszynski (bass) and Stefan Schwarzmann (drums).

Even though the unwritten U.D.O. law No-Keyboards was broken, U.D.O. achieved an amazing success, with the album Faceless World, produced by ex-Accept drummer Stefan Kaufmann.

At first disputed, it turned out to be, with Udo Dirkschneider (vocals), Mathias Dieth (guitar), Wolla Boehm (guitar), Thomas Smuszynski (bass) and Stefan Schwarzmann (drums), U.D.O.'s most sold album so far.