au

The network that would eventually become au was originally set up as two networks: DDI and IDO. IDO's network was based upon the NTT Hi-cap analog cellular system, and began operations in December 1988 in the Kanto and Tokai regions. DDI's network was run by independent phone companies, and began service in 1989 using the TACS system elsewhere in Japan. Nippon Idou Tsushin (IDO) was owned by Toyota, whereas DDI was owned by Kyocera Corporation.

au K.K. (株式会社エーユー) was established in November 2000 by Kyocera as part of the DDI Cellular network. In 2001, the company was merged into KDDI (which had been formed in 2000 by the merger of DDI, KDD, and IDO), but its brand name was retained and applied to all mobile phone service under KDDI group.

au established a nationwide 3G network in 2003, replacing its previous cdmaOne service with CDMA 1X WIN (1xEV-DO Rev A) service. It started selling the iPhone 4S from 14 October 2011. The company launched LTE service as 'au 4G LTE' in September 2012. It launched 5G service as 'UNLIMITED WORLD au 5G' on 26 March 2020.

au sponsored the Cerumo team in Japan Grand Touring Championship between 2001 and 2004, and has gone on to become the title sponsor of the #36 TOM'S entry in its successor Super GT from 2016 to present.