<p>Cities were once considered population sinks for many species, but it is now clear that they can offer viable and even high-quality animal habitats, and thus have an essential role in maintaining global biodiversity. In some ways, cities are island-like habitats connected by human infrastructure, governed by the rules of island biogeography. Yet cities are also unique mosaic landscapes with heterogeneity that reflects both the local, natural ecosystem and an urban landscape common to cities worldwide. Species-specific evolutionary and ecological histories determine how species interact with different features of the landscape and thus the patterns of urban colonization and persistence. In this Review, we explore how ecological and evolutionary processes play out in the urban mosaic and shape urban species assemblages. Accounting for the interplay of complex ecological and evolutionary mechanisms on multiple, hierarchical spatial scales will enhance understanding of how urban biodiversity is accumulated and maintained, and enable better management of species as cities expand and intensify globally.</p>