In tropical forest ecosystems, majority of plant species depend on frugivorous vertebrates for seed dispersal. Because primates constitute a large portion of frugivore biomass in neotropical and paleotropical forests, the roles of primates as seed dispersers have been examined since 1970's. The process of seed dispersal by vertebrates can be divided into three phases: (1) pre-dispersal phase in which animals select particular fruits as attractive food; (2) dispersal phase in which animals handle, transport, and deposit seeds during their foraging activities; (3) post-dispersal phase in which dispersed seeds germinate and grow to reproductive age. To understand roles of primates as seed dispersers, this paper marshals the previous achievements in order of the three phases. During pre-dispersal and dispersal phases, quantitative and qualitative effectiveness of seed dispersal are generally related to several anatomical traits and behavioral pattern of each primate taxon, and particularly, large-bodied frugivorous primates often perform effectively. However, during post-dispersal phase, high mortality and unpredictable fates of dispersed seeds dramatically hurt the effectiveness of primate seed dispersal. Large-bodied frugivorous primates are recognized as vulnerable taxa to human disturbance such as deforestation and bushmeat hunting. Recently, in the forests where such primates are locally extinct or reduced, researchers have demonstrated that the loss of their seed dispersal services drives low density of seedlings and saplings and low rates of gene flow among populations. These facts paradoxically suggest that primates contribute regeneration of plant populations even their effectiveness is lessened in post-dispersal phase. As future issues, integration between seed dispersal research and plant demographic study will develop our understanding of the primates' roles in plant population dynamics over many generations. Moreover, considering the critical situations in empty forests, there is also an urgent need to argue prioritized conservation of high-performance primates for maintaining regeneration and vegetative restoration in degraded forests.