Identifying and quantifying sex differences and similarities has been a central research question and fascinated scientists for centuries. A large body of work has been accumulated on this topic; however, conclusions are often drawn as if they are applicable across cultures even though studies have predominantly relied on Western samples. This chapter reviews cross-cultural literature on several early childhood sex differences in domains of development that have caught attention in the literature recently: gender-typed play, gender identity, and gender expression. We also offer an overview of possible influences on sex differences, including evolutionary, biodevelopmental (genetics, sex hormones, and immune factors), and sociocultural mechanisms (socialization and macro-cultural factors). Given that a cross-cultural perspective has often been lacking in this literature, this chapter reviews research on early gender development in males and females from Western populations as well as the non-Western populations wherever possible to highlight important cultural (in)consistencies.link_to_subscribed_fulltex