German heavy metal band formed by former Accept vocalist Udo Dirkschneider in 1987.The band is closely related to Accept, founded by Udo Dirkschneider and Michael Wagener in Solingen, Germany.

After the split of Accept in 1987, a large fan base followed Udo Dirkschneider to U.D.O.

Beginning with their first albums, Animal House (1988) and Mean Machine (1989), U.D.O. played pure metal without any impurities. Animal House featured Peter Szigeti (guitar), Frank Rittel of Warlock

(bass guitar), Mathias Dieth of Sinner (guitar), Thomas Franke (drums) and, of course, Udo Dirkschneider (vocals).

With the release of Mean Machine in 1989, the band had a totally different look. The line-up of Animal House was down to Mathias Dieth (guitar), apart from Udo. The new members were Andy Susemihl (guitar), Thomas Smuszynski (bass guitar) and Stefan Schwarzmann (drums).

Despite breaking the unwritten rule of not using keyboards, U.D.O. found great success with the album Faceless World, which was produced by former Accept drummer Stefan Kaufmann. It became U.D.O.'s best-selling album to date. The album was recorded by Udo Dirkschneider (vocals), Mathias Dieth (guitar), Wolla Böhm (guitar), Thomas Smuszynski (bass guitar) and Stefan Schwarzmann (drums). In 1991, after the release of the album Timebomb (Frank Fricke replacing former guitarist Wolla Böhm), Udo Dirkschneider temporarily suspended the band's activities in 1992 in order to revive Accept.

In1996, Accept finally disbanded, and U.D.O. was revived. Udo Dirkschneider invited Stefan Kaufmann (guitar), Jürgen Graf (guitar), Fitty Wienhold (bass guitar) and Stefan Schwarzmann (drums). The song Metal Gods was then recorded for the Tribute to Judas Priest set.Solid was released in 1997 and No Limits in 1998.

The line-up of U.D.O. (Udo Dirkschneider (vocals), Stefan Kaufmann (guitar), Lorenzo Milani (drums), Igor Gianola (guitar), and Fitty Wienhold (bass guitar)) changed and released the album Holy in 1999. U.D.O. remained in this line-up until 2004.