The Audi A4 is a compact executive car produced by the German car manufacturer Audi AG since late 1994. The initial generation A4 succeeded the four previous generations of the Audi 80. And just like its Audi 80 predecessor, the Audi A4 automobile layout consists of a longitudinally oriented engine at the front, with transaxle-type transmissions mounted immediately to the rear of the engine, again, oriented longitudinally. Drive output is transmitted to either the front wheels, or on some models, utilising Audi's 'trademark' quattro permanent four-wheel drive system via a Torsen centre differential.

Since its inception, the Audi A4 has been available in four generations, based on the Volkswagen Group 'B' series of automobile platforms. Audi's internal numbering convention therefore treats the A4 as a logical continuation of the Audi 80 lineage, hence the initial A4 was designated as the B5-series, followed by the subsequent B6 and B7, and the current B8.

Of the main body styles, it is (and always has been) available as a saloon/sedan variant, and an "Avant", which is Audi's name for an estate/wagon. The second (B6) and third generations (B7) of the A4 also had a Cabriolet version, but the Cabriolet was not retained for the current fourth generation (B8), Audi now having Cabriolet versions of the related Audi A5 and S5 coupés - and also the smaller Audi A3.

Additionally, higher performance versions directly related to the A4 include the Audi S4, and the limited availability quattro GmbH-developed very high performance Audi RS4.