At around three years of age, children begin to show a preference for same-sex peer affiliations. This gender segregation occurs in all cultures where children"s social groups are large enought to allow choice, and it appears to have important influences on children"s development. Different peer group environments may lead to the development of different psychological preferences and skills. They may also foster later gender differences in academic achievement and intimacy. The contirbutors to this volume of New Directions for Child Development examine both the possible developmental precursors and consequences of gender segregation, implicationg social, emotional, physiological, and cognitive factors in the emergence and maintenance of individuals" preferences for same-sex peer groups. This is the 65th issue of the journal series New Directions for Child Development.