The BMW E21 is the first generation of the BMW 3 Series compact executive cars, which were produced from June 1975 to 1983 and replaced the BMW 02 Series. The series was exclusively built in a two-door sedan body style (except for convertibles made by Baur). Contrary to its predecessor, a 'Touring' body with a sloped rear hatch was no longer offered.

Initial models were produced with carburetted four-cylinder petrol engines of 1.6 L, 1.8 L, and 2.0 L. Fuel-injection was introduced in late 1975 on the 320i – but in 1977, a carburetted inline 6-cylinder engine replaced both the 320 and 320i models, while a detuned version of the four-cylinder injection went on sale in the United States. In 1978, the 3-series received its first fuel-injected six-cylinder in the 323i; and in 1981, a bottom end detuned 1.6 L was offered in the 315.

The cabriolet body style, manufactured by Baur, was produced from 1978 to 1981, with all available engines.