Sex segregation (the separation of boys and girls into same-sex groups in their friendships and casual encounters; [Thorne, B., & Luria, Z. (1986). Sexuality and gender in children's daily worlds. "Social Problems, 33", 176-190]), has typically been viewed as a childhood phenomenon. In the present paper, we review research that suggests the phenomenon of sex segregation persists across the life span. We draw from a social-constructionist model to synthesize literature documenting sex segregation in friendships and aspects of individuals' socio-cultural contexts in childhood, adolescence, and early and later adulthood. We consider developmental continuities in mechanisms that may be associated with sex segregation, including behavioral compatibility, communication styles, third-party resistance to other-sex relationships, and institutional barriers to other-sex relationships. We discuss how these factors may, through a reciprocal process, contribute to and result from sex segregation. Finally, we offer recommendations for future research.