Akershus is the second largest county by population after Oslo, with more than half a million inhabitants. The county is named after Akershus Fortress. The county administration is in Oslo, which is not part of the county per se.

The county is conventionally divided into the traditional districts Follo and Romerike, which fill the vast part of the county, as well as the small enclave west of Oslo that consists of Asker and Bærum. This became the result after the transferring of the great municipality of Aker (surrounding Oslo) from Akershus county to Oslo in 1948.

Embracing numerous suburbs of Oslo, notably Bærum, Akershus is one of the most densely populated areas in the country. The main national railway lines into Oslo run through Akershus with many junctions and stations such as Asker, Sandvika, Ski and Lillestrøm. Akershus includes some of the lake Mjøsa and some of the river Glomma.