Associated Equipment Company (AEC) was a British vehicle manufacturer that built buses, motorcoaches and trucks from 1912 until 1979. The name Associated Equipment Company was hardly ever used; instead, it traded under the AEC and ACLO brands. During World War One, AEC was the most prolific British lorry manufacturer, after building London's buses before the war.

The London General Omnibus Company (LGOC) was founded in 1855 to amalgamate and regulate the horse-drawn omnibus services then operating in London. The company began producing motor omnibuses for its own use in 1909 with the X-type designed by its chief motor engineer, Frank Searle, at works in Blackhorse Lane, Walthamstow. The X-type was followed by Searle's B-type design, considered to be one of the first mass-produced commercial vehicles.

In 1912, LGOC was taken over by the Underground Group of companies, which at that time owned most of the London Underground, and extensive tram operations. As part of the reorganisation following the takeover, a separate concern was set up for the bus manufacturing elements, and was named Associated Equipment Company, better-known as AEC.

AEC's first commercial vehicle was a lorry based on the X-type bus chassis. With the outbreak of World War I in 1914, AEC's ability to produce large numbers of vehicles using moving-track assembly lines, based on American principles, became important in supplying the increasing need for army lorries. AEC commenced large-scale production of their 3-ton Y-type lorry in 1916, including some with Daimler-built engines, badged and supplied as Daimler. Continued beyond the end of the war, over 10,000 Y-type lorries were manufactured – making AEC Britain's largest domestic provider of lorries for the nation's military. From then on, AEC became associated with both lorries and buses.

In 1926, AEC and Daimler formed the Associated Daimler Company, which was dissolved two years later. In 1927, AEC moved its manufacturing from Walthamstow to a new plant at Southall.

G. J. Rackham was appointed Chief Engineer and Designer in 1928. He had previously worked for Leyland Motors. His ideas contributed significantly to AEC's reputation for quality and reliability.

From 1929, AEC produced new models: the names of lorries began with "M" (Majestic, Mammoth, Mercury, and so on), and those of buses began with "R" (Regent, Regal, Renown, and so on). These original "M-models" continued in production until the end of World War II. AEC introduced diesel engines across the range in the mid-1930s.

From 1931 to 1938, AEC and English Electric co-produced trolleybuses. AEC supplied the chassis, and English Electric the electric motors and control equipment.