I report on the social and breeding biology of four bee-eater species in Thailand. Little Green Bee-eaters (Merops orientalis) breed cooperatively in clusters of overlapping territories. Cooperative breeding units have one to two helpers that join the breeding pair only after incubation has begun. Nests rarely are left unguarded due to threats of predation and possible intraspecific brood parasitism. Males also guard their mates against extrapair copulations. The Blue-tailed Bee-eater (M. philippinus) breeds cooperatively and has a complex social system, with evidence suggestive of intraspecific brood parasitism and extrapair copulation. I provide evidence that the Bay-headed Bee-eater (M. leschenaulti) breeds cooperatively and report observations of noncooperative breeding at one nest in the Blue-bearded Bee-eater (Nyctyornis athertoni).