Interspecific brood parasitism, in which females lay eggs in the nests of other species, has been observed in many altricial birds. In addition, nestling aggression, in which parasite nestlings attempt to out compete or kill host eggs or nestlings, occurs in many species of interspecific brood parasites. We examine the relationship traits for nestling aggression potentially have to the evolution of obligate brood parasitism. We propose the following: (1) under conditions in which a low number of extra eggs is made possible by being freed from parental care, a stable facultative stage could arise; (2) if aggression arose in a stable facultative parasite, aggression would be selected for if the ratio of the increase in success conferred on young in host nests to the decrease in success suffered by young in the home nest was above some threshold; and (3) once aggression had arisen in the facultative parasite, it could produce strong selection for the evolution of an obligate strategy.