Invasive species can alter resident communities through an enormous number of potential processes. The challenge is to determine the causal mechanisms behind such community changes and synthesize them into a predictive framework. The study of animal behavior offers a potential path to gain a mechanistic understanding of the invasion process, because behavior can affect invasions at several stages and in diverse ways. Behavioral flexibility, boldness, or fear can affect dispersal, colonizing success, and predator-prey interactions. Foraging behavior and social interactions can affect exploitation or monopolization of a shared resource. Overlap in species or mate recognition cues can affect hybridization or reproductive interference. Thus, behavior can play a key role in determining which species are likely to invade and how they may affect resident species. Twenty years ago, several ecologists called for a more mechanistic approach to community ecology. For animals, a mechanistic approach often involves understanding how individual behavior affects species interactions. Mechanistic research at this level of organization has grown steadily but slowly. Stimulating more research into the behavioral mechanisms of invasion will therefore address a longstanding goal of community ecology.