Abstract Cities occupy about 3 per cent of the Earth’s habitable land area and are home to one out of two humans worldwide; both estimates are predicted to grow. Urban space is thus becoming an important, novel ecological niche for humans and wildlife alike. Building on knowledge gathered by urban ecologists during the last half century, evidence of evolutionary responses to urbanization has rapidly emerged. Urban evolutionary biology is a nascent yet fast-growing field of research—and a fascinating testing ground for evolutionary biologists worldwide. Urbanization offers a great range of opportunities to examine evolutionary processes because of the radically altered and easily quantifiable urban habitat, and the large number of cities worldwide, enabling rigorous, replicated tests of evolutionary hypotheses. Urban populations are increasingly exhibiting both neutral and adaptive evolutionary changes at levels ranging from genotypes to phenotypes. The novelty of urban evolutionary biology is that these changes are driven by the cities we have built, including effects of infrastructure, pollution, and social characteristics of our urban neighbourhoods. It will thereby enrich the field of evolutionary biology with emergent yet incredibly potent new research themes where the urban habitat is key. In a series of sixteen chapters written by leading evolutionary biologists working on urban drivers of evolution, Urban Evolutionary Biology is the first academic book in the field. It synthesizes current knowledge on evolutionary processes occurring literally on our doorstep, across the globe, and in each city independently.