The OECD, in cooperation with the EU, has developed a harmonised definition of 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 (OECD, 2012). The definition of FUA aims at providing a functional/economic definition of cities and their area of influence, by maximising international comparability and overcoming the limitation of using purely administrative approaches. At the same time, the concept of FUA, unlike other approaches, ensure a mimimum link to the government level of the city or metropolitan area. The Metropolitan database provides socio-economic and environmental indicators for around 1 300 OECD functional urban areas over 50,000 inhabitants in 37 OECD countries. Some indicators are only available for large FUAs (i.e. above 250 000 inhabitants). Most of the indicators presented in the database are modelled based on aggregation of data at smaller geographic scale (e.g. population by age). Other indicators are modelled using geo-spatial data sources (e.g. air quality) or by downscaling/adjusting indicators available at slightly different geographic scale thorugh the use of population grid (e.g. GDP). FUAs are defined in several steps. First, a population grid makes it possible to define ‘urban centres’ independently from administrative or statistical boundaries. An urban centre is a pure grid-based concept, a cluster of contiguous cells of high density and with more than 50,000 inhabitants. Subsequently, this dense, urban centre is adapted to the closest local units to define a city (or ‘core’). Next, commuting flows are used to identify which of the surrounding, less densely populated local units were part of the city’s labour market (commuting zone). Commuting zones are defined as all municipalities with at least 15% of their employed residents working in a certain city core. Municipalities surrounded by a single functional urban area are included and non-contiguous municipalities are dropped. See documents in the reference list for further details. Local units used as building blocks to define FUAs are different across countries. In the case of European countries they are Local Administrative Units (LAU) according to the terminology adopted by Eurostat. In most cases local units are municipalities. In the case of the United States, census tracts are used for the method, and the final boundaries of the city and the commuting zones are adapted to the county boundaries.