Brood parasitism involves the exploitation of host parental care rather than the extraction of resources directly from hosts. Although often excluded, we show that brood parasitism fits within existing frameworks of parasitic strategies, and formulate defining criteria, including that the parasites be obligate and the hosts non-eusocial (thereby distinguishing them from social parasites). A systematic literature survey revealed 59 independently derived brood parasitic lineages with most origins (49) in insects, particularly among bees and wasps, and other origins in birds (7) and fish (3). Insects account for over 98% of brood parasitic species, with much of that diversity reflecting ancient (100+ million-year-old) brood parasitic lineages. Brood parasites usually, but not always, evolve from forms that show parental care. In insects, brood parasitism often first evolves through exploitation of a closely related species, following Emery’s rule, but this is less typical in birds, which we discuss. We conducted lineage-level comparisons between brood parasitic clades and their sister groups, finding mixed results but an overall neutral to negative effect of brood parasitism on species richness and diversification. Our review of brood parasites reveals many unanswered questions requiring new research including further modelling of the co-evolutionary dynamics of brood parasites and their hosts.