In many primate species, more males than females leave their natal group and transfer to another. In man, chimpanzee and the gorilla, however, the reverse is the case. This paper presents detailed data for the gorilla on individuals' movements into and out of breeding units. Comparisons are made with other primates, and with data on two non-primate species in which females rather than males move between breeding units. Proximate causes and functions of emigration and transfer are considered, and the reasons (proximate and evolutionary) for the observed sex differences in frequency of movement are discussed.