Advances in molecular genetic techniques have provided new approaches for addressing evolutionary questions about brood parasitic birds. We review recent studies that apply genetic data to the systematics, population biology, and social systems of avian brood parasites and suggest directions for future research. Recent molecular systematics studies indicate that obligate brood parasitism has evolved independently in seven different avian lineages, a tally that has increased by one in cuckoos (Cuculiformes) and decreased by one in passeriforms (Passeriformes) as compared to conventional taxonomy. Genetic parentage analyses suggest that brood parasitic birds are less promiscuous than might be expected given their lack of nesting and parental care behavior. Host-specificity in brood parasites, which has important implications for host-parasite coevolution, has been evaluated using both population genetic and parentage analyses. Female lineages are faithful to particular host species over evolutionarily significant time scales in both common cuckoos (Cuculus canorus) and indigobirds (Vidua spp.), but differences in the host-specificity of male parasites has resulted in different patterns of diversification in these two lineages. Future research on brood parasitism will benefit from the availability of comprehensive molecular phylogenies for brood parasites and their hosts and from advances in functional genomics.