Lillestrøm is a municipality in Akershus county. It is located in the traditional district of Romerike. With a population of 85,757 inhabitants, it is the fourth most populated municipality in Viken. It was founded on 1 January 2020 as a merger between former municipalities Fet, Skedsmo and Sørum. The administrative centre of the municipality is the town of Lillestrøm. The town of Lillestrøm is a part of the Oslo metropolitan area.

Akershus is a county in Norway, with Oslo as its administrative centre. Akershus has been a region in Eastern Norway with Oslo as its main city since the Middle Ages, and is named after the Akershus Fortress in Oslo and ultimately after the medieval farm Aker in Oslo. From the Middle Ages to 1919, Akershus was a main fief and main county that included most of Eastern Norway, and from the 17th century until 2020 and again from 2024, Akershus also has a more narrow meaning as a smaller central county in the Greater Oslo Region. Akershus is Norway's largest county by population with over 716,000 inhabitants.