Book excerpt: Cities are not only home to around half of the global population but also major centers of economic activity and innovation. Yet, so far there has been no consensus of what a city really is. Substantial differences in the way cities, metropolitan, urban, and rural areas are defined across countries hinder robust international comparisons and an accurate monitoring of SDGs. The report Cities in the World: A New Perspective on Urbanisation addresses this void and provides new insights on urbanisation by applying for the first time two new definitions of human settlements to the entire globe: the Degree of Urbanisation and the Functional Urban Area.