The urbanization process contributes to a general impoverishment of biodiversity, yet the underlying reasons remain poorly understood. The classical explanation is that organisms differ in tolerance to urbanization, but current supporting evidence is insufficient because of the difficulties of quantifying such tolerance accurately in comparative studies. However, with information on the abundance of species along urbanization gradients, it is possible to construct simple urbanization tolerance indices that account for the fact that the density of a species in an urbanised environment depends in part on its density in the surrounding natural environments. The use of such metrics is likely to change our perception of how animals respond to the urbanization process, allowing a better investigation of whether urban communities are non-random assemblages of species and, if so, whether such non-random patterns are caused by adaptations that make some species more tolerant to urbanization than others.