Contemporary urban systems in OECD countries are structured around functional regions, which often overcome established city boundaries. Reading space in terms of functional regions allows assessing changes in urban hierarchies and spatial structures, including the polycentricity of urban systems at national, regional and metropolitan scale. By using a harmonised definition of functional urban areas in OECD countries, this paper first provides a sound definition of polycentricity at each spatial scale, highlighting for each of them the different links with policy. Second, it provides measures of polycentricity and explores the economic implications of different spatial structures. Results show that relatively more monocentric regions have higher GDP per capita than their more polycentric counterparts. At country level, on the other hand, polycentricity is associated with higher GDP per capita.