Mutualistic interactions are diverse and widespread and often involve multispecies guilds of mutualists competing for access to one or more partner species. Despite the ubiquity of these interactions, we know little about the dynamics of competition and coexistence within these guilds or how interactions between mutualists and their shared resource (the partner species) may influence these dynamics. In this article, we review the evidence for interspecific competition for partners within mutualist guilds in both plant-pollinator and ant-myrmecophyte systems. We then review evidence for the operation of different coexistence mechanisms within these guilds and discuss how the dynamics of competition and coexistence may be uniquely shaped by multispecies mutualist interactions. In particular, we note that adaptive and plastic responses by mutualists to variation in partner quality are likely to play an important role in determining these dynamics. We summarize by considering the ecological conditions that are likely to restrict or promote species coexistence within mutualist guilds. We suggest that these guilds may provide powerful model systems for exploring multiple mechanisms of species coexistence, and we discuss how these mechanisms may be modified by evolutionary adaptation.