Abstract In classical polyandry, sex roles are reversed and a female reproduces with several males, each of whom raises his offspring with little or no help from her. This mating system occurs in some fishes and birds, and it is of great interest in relation to parental investment, sex role and sexual selection theory. The evolution of classical polyandry, however, is debated and not well understood. It is here suggested to generally take place in three main steps. (1) First evolves male care for eggs, for reasons that differ between fishes and birds. (2) Second, a female becomes able to lay more eggs than a male can accommodate. This can happen, for example, by evolution of male pregnancy or smaller body size, or by female production of more or larger eggs, made possible by larger female body size or more food. Polyandry in several taxa is associated with shift to a habitat rich in food during the breeding season, to novel specialised foraging methods, or to both. A favourable food situation may be crucial for evolution of classical polyandry. (3) In step three, females compete to lay two or more clutches in sequence for different males. Successful polyandrous females obtain more offspring, spreading traits that enhance success in competition over males. Step three may be most likely in species with small body size, for reasons of reproductive constraints and seasonality. Evolution of classical polyandry appears to have followed these steps in shorebirds, coucals and pipefishes, but the reasons why certain species differ from their close phylogenetic relatives in being polyandrous are far from clear. Behavioural and ecological studies of additional species, and detailed phylogenies of taxa with diverse mating systems including polyandry, are needed for testing these ideas.