This chapter describes the mating systems of bats, emphasizing research from the last 20 years. For completeness, the chapter lists all species where mating systems are defined, but it discusses only those where there is new information or where additional interpretations are possible. In total, it includes information on the mating systems of 66 species of bats in 10 families. Bats from the largest families (Vespertilionidae, Pteropodidae, Phyllostomidae) are best represented in this literature. In proportion to their species diversity, bats in the Rhinolophidae and Molossidae have received the least attention. Bats from the zoogeographic regions with the greatest numbers of species (Ethiopian, Neotropical, Oriental) also have received the most study. In proportion to regional species richness, mating systems have been least studied in bats from the Australian and Oriental regions. Combined with detailed field studies, the immediate future is likely to see an increasing emphasis on the use of these new molecular methods to resolve the big issues in bat mating systems. It is anticipated that these issues are the questions of apparent monogamy, the significance of multiple mating by females, and the roles of female choice and sperm competition.