This paper describes the EU-OECD method to define functional urban areas (FUAs). Being composed of a city and its commuting zone, FUAs encompass the economic and functional extent of cities based on daily people’s movements. The paper first presents briefly the methodological approach and subsequently provides a detailed description of the identification algorithm, together with the data needed to apply it. This definition has been applied to 33 OECD member countries and Colombia, as well as to all European Union member countries.