KDDI Corporation is a Japanese telecommunications operator. It was established in 2000 through the merger of DDI , KDD (ケイディディ), and IDO . In 2001, it merged with a subsidiary named Au, which was formed through the merger of seven automotive and mobile phone companies from the DDI-Cellular Group. As of 2020, it is the second-largest mobile telecommunications provider in Japan in terms of the number of contracts, following NTT Docomo.

au, or au by KDDI, is a Japanese mobile phone operator. au is a brand marketed by KDDI in the main islands of Japan and by Okinawa Cellular in Okinawa for their mobile cellular services. au is the second-largest wireless carrier in Japan, with 60.398 million subscribers as of March 2021.