Gender is a highly salient and important social group that shapes how children interact with others and how they are treated by others. In this Element, we offer an overview and review of the research on gender development in childhood from a developmental science perspective. We first define gender and the related concepts of sex and gender identity. Second, we discuss how variations in cultural context shape gender development around the world and how variations within gender groups add to the complexity of gender identity development. Third, we discuss major theoretical perspectives in developmental science for studying child gender. Fourth, we examine differences and similarities between girls and boys using the latest meta-analytic evidence. Fifth, we discuss the development of gender, gender identity, and gender socialization throughout infancy, early childhood, and middle childhood. We conclude with a discussion of future directions for the study of gender development in childhood.