Obligate brood-parasitic cheats have fascinated natural historians since ancient times. Passing on the costs of parental care to others occurs widely in birds, insects and fish, and often exerts selection pressure on hosts that in turn evolve defences. Brood parasites have therefore provided an illuminating system for researching coevolution. Nevertheless, much remains unknown about howecology and evolutionary history constrain or facilitate brood parasitism, or the mechanisms that shape or respond to selection. In this special issue, we bring together examples fromacross the animal kingdomto illustrate the diverseways in which recent research is addressing these gaps. This special issue also considers how research on brood parasitism may benefit from, and in turn inform, related fields such as social evolution and immunity. Here, we argue that progress in our understanding of coevolution would benefit from the increased integration of ideas across taxonomic boundaries and across Tinbergen's Four Questions: mechanism, ontogeny, function and phylogeny of brood parasitism. We also encourage renewed vigour in uncovering the natural history of the majority of the world's brood parasites that remain little-known. Indeed, it seems very likely that some of nature's brood parasites remain entirely unknown, because otherwise we are left with a puzzle: if parental care is so costly, why is brood parasitism not more common? This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.Non peer reviewe
