Lemurs, lorises, and tarsiers are socially and ecologically diverse primates that include some of the most endangered mammals. We review results of long-term studies of 15 lemur species from 7 sites in Madagascar and 1 species each of loris and tarsier in Indonesia. We emphasize that the existence of long-term study populations is a crucial prerequisite for planning and conducting shorter studies on specific topics, as exemplified by various ecophysiological studies of lemurs. Extended studies of known individuals have revealed variation in social organization within and between ecologically similar species. Even for these primates with relatively fast life histories, it required more than a decade of paternity data to characterize male reproductive skew. The long-term consequences of female rank on reproductive success remain poorly known, however. Long-term monitoring of known individuals is the only method to obtain data on life-history adaptations, which appear to be shaped by predation in the species covered here; long-term studies are also needed for addressing particular questions in community ecology. The mere presence of long-term projects has a positive effect on the protection of study sites, and they generate unique data that are fundamental to conservation measures, such as close monitoring of populations.