In this article we review the current knowledge of the effects of urban expansion on bats and assess the potential of these mammals as bioindicators of urbanization. The response of bats to this process is highly species-specific: some species tolerate urban habitat or are even favoured by its roosting or foraging opportunities, others are affected by the loss or fragmentation of key natural habitat, or by the physical and chemical pollution associated with urbanization. Species responses generally translate into altered community structures, with few markedly dominating species. We propose different hypothetical models of bat fitness along an urbanization gradient and discuss why bat population density may not be an effective fitness proxy to assess the reactions of these mammals to urban expansion. We also suggest that urban habitat may act as an ecological trap even for apparently synurbic species. Overall, bat sensitivity to urbanization makes these mammals promising candidates to track the effects of this process of land use change on the biota, but more studies, specifically tailored to explore this role, are needed.