In this chapter, the different meanings of the terms sex and gender are discussed: Sex is biological, and gender has to do with social roles. Biological differences such as genes are discussed next, including a discussion of whether these differences should be considered as either/or distinctions or as continuums. Differences in social skills are discussed. Next, differences in the brain’s gray and white matter are explored. Various parts of the brain and the abilities they support are then presented. How sex differences in the brain complement each other is explored, as well as differences and overlaps. The implications for single-sex education are presented. Reasons to discuss brain differences and other differences follow, including sleep problems, anorexia, and bulimia. A subsection on memory and emotion follows.