The world is urbanising with 70% of the world’s population expected to live in urban areas by 2050 (UN, 2009). Today, two-thirds of the OECD population live in urban areas, according to the OECD-EC definition.

The 275 metropolitan areas in OECD countries accounted for 48% of OECD population, 56% of the total gross domestic product (GDP) and 49% of employment in 2010. The concentration of population and GDP ranges from 70% in Japan to less than 30% in the Slovak Republic ().

The 275 OECD metropolitan areas (with populations of at least 500 000) contributed on average to over half of the total OECD growth over the period 2000-10.

Metropolitan areas drive national employment creation in many countries. On average, half of overall employment creation in 22 OECD countries between 2000 and 2012 was accounted for by 232 metropolitan areas. The metropolitan contribution to national employment growth was parti-cularly high in Korea and Canada (more than 70%), while metropolitan areas in the Slovak Republic and Italy contributed to less than 35% of national employment growth ().

In many countries, the difficult labour market conditions resulting from the economic crisis have been persistent also in metropolitan areas. The unemployment rate in metropolitan areas rose more in the period 2008-2012 than it did in the previous 8 years in 26 of the 28 OECD countries (). In Athens and Thessaloniki (the two metropolitan areas of Greece), the unemployment rate increased on average 5 percentage points annually between 2008 and 2012, reaching 25% of unemployed in 2012 ().

Innovation is highly concentrated in a few countries, and metropolitan areas are usually the places where most innovation activities take place. Agglomeration forces determine an environment with a large proportion of specialised workers, firms and capital, where ideas are easily exchanged and can lead to the creation of new goods and production processes. In 2008, 65% of all patent applications of the 16 OECD countries where data are available were granted in metropolitan areas (). The concentration of patents in metropolitan areas is high in top patenting countries such as Japan and the United States but also in France, the Netherlands, Spain and Denmark. On the other side, Finland, Norway and Italy displayed a lower share of patents granted by metropolitan areas, signalling innovation activities outside the capital areas of Helsinki (e.g. in Pirkanmaa and Pohjois-Pohjanmaa) and Oslo (e.g. in Rogaland, Hodaland and Sor-Trondelag) as well in medium-sized cities in northeast Italy.

Green areas such as parks and natural vegetation contribute to reducing pollution, improving the health and quality of life of residents, and making metropolitan areas more attractive to residents and tourists.

Metropolitan areas are continuously changing their spatial organisation, reflecting the evolution of economy and society. These changes affect the quality of life, the demand for transport infrastructure, and the global environmental footprint of urbanisation, among other factors. Regional, metropolitan and local governments’ decisions depend critically on the physical structure of the city. On average, 80% of the OECD urban population lives in the cores of metropolitan areas and only 20% in the hinterlands, but in a few European countries the share of population in urban cores is below 50% (). While most of the metropolitan areas have grown with contiguous urban cores, 30 metropolitan areas show a polycentric structure with more than one urban core.