The provision of care to non-offspring in animal societies has attracted substantial scientific attention because of its apparent contradiction to the concept of individual selection. In mammals such cooperative care of alien young has been described for both non-breeding and breeding females. I first review the types of non-offspring care that are known from mammals and then discuss the actual and potential fitness benefits and costs for the donors and recipients of this cooperative behaviour. For many species, however, quantitative analysis is still missing. Non-offspring care provided by non-breeding individuals may best be explained by indirect fitness benefits due to improved reproduction of a related breeder under environmental conditions in which successful direct reproduction is not possible. Cooperative care of young among breeding females is also directed preferentially to non-descendant kin and may have evolved due to mutualistic benefits. Our current knowledge of mammalian cooperative care of the young raises questions that must be answered in order to fully understand the evolution of social behaviour.