Abstract: Exploring the mechanisms underlying community species richness is a key issue in ecology and conservation biology, and many hypotheses based on small-scale, local processes have traditionally been used as explanations. The species pool hypothesis developed by Zobel et al. suggests that the variation in community species richness is not only associated with contemporary environmental factors and ecological processes (e.g. competition and predation), but also limited by the regional species pool. The regional species pool is the set of species in a certain region that are capable of coexisting in a target community, which is shaped by historical (e.g. glaciation and geological age) and regional processes (e.g. speciation, immigration, dispersion, and extinction). The species pool hypothesis suggests that the larger the area of a habitat type and the greater its geological age, the greater the opportunity for speciation and hence the larger the number of available species adapted to that particular habitat, which will in turn lead to higher community diversity. The species pool is generally studied at two spatial scales: the regional and the actual scales. While the regional species pool is primarily determined by biogeographic processes, the actual species pool (species present in the target community) is determined by both ecological processes (e.g. competition) and the regional pool. In this review, we introduce and discuss the concepts relating to, and evidence for the species pool hypothesis, together with methods for estimating the species pool.