Mutualisms – cooperative interactions among different species – are known to influence global biodiversity. Nevertheless, theoretical and empirical work has led to divergent hypotheses about how mutualisms modulate diversity. We ask here when and how mutualisms influence species richness. Our synthesis suggests that mutualisms can promote or restrict species richness depending on mutualist function, the level of partner dependence, and the specificity of the partnership. These characteristics, which themselves are influenced by environmental and geographic variables, regulate species richness at different scales by modulating speciation, extinction, and community coexistence. Understanding the relative impact of these mechanisms on species richness will require the integration of new phylogenetic comparative models as well as the manipulation and monitoring of experimental communities and their resulting interaction networks.