As of 2010, 4,231 cities and metropolitan areas had 100,000 or more people. To better understand these cities, the Monitoring Urban Expansion initiative tracked a global stratified sample of 200 cities in 1990, 2000, and 2014, collecting and analyzing data on the quantity of land urbanized through expansion and on selected characteristics of that expansion. 

This initiative has partnered with the United Nations Human Settlements Programme (UN-Habitat) and the Lincoln Institute of Land Policy to produce the Atlas of Urban Expansion, a flagship publication that demonstrated several key findings: urban population densities are declining by 2.1% per year, newly built areas on the urban periphery are less likely to be properly planned than older areas, and cities are becoming less walkable and less transit-friendly.

Following the completion of the Atlas, additional regional and national analyses have generated valuable information on urban expansion in over 400 cities in 78 countries.