Sex differences in attachment styles have been found in adulthood, emerge as early as middle childhood, and can be sizable when described at the appropriate level of analysis. However, they have received relatively little attention in mainstream attachment research. Here I review the evidence of sex differences in attachment, including what is currently known about developmental patterns and cross-cultural variation. I summarize existing evolutionary models of sex differences, and discuss evidence for a role of prenatal and postnatal sex hormones. I highlight current theoretical and empirical gaps in the literature, and call for more integrative research on this fascinating topic.