Bayer 04 Leverkusen Fußball GmbH, also known as Bayer 04 Leverkusen, Bayer Leverkusen, or simply Bayer, is a German professional football club based in Leverkusen in the state of North Rhine-Westphalia. The club competes in the Bundesliga, the top tier of German football, and plays its home matches at the BayArena.
The club was founded in 1904 by employees of the German pharmaceutical company Bayer AG, whose headquarters are in Leverkusen and from which the club draws its name. It was formerly the best-known department of TSV Bayer 04 Leverkusen, a sports club whose members also participate in athletics, gymnastics, basketball and other sports including the RTHC Bayer Leverkusen. In 1999 the football department was separated from the sports club and is now a separate entity formally called Bayer 04 Leverkusen GmbH. Bayer were first promoted to the Bundesliga in 1979, and have remained in the top division ever since. The club's main colours are red and black, both having been used as the main shirt colour and with red and black stripes also having been used as home colours.