Men have higher mortality rates, but women have higher morbidity rates. This chapter discusses this paradox and focuses on the explanations for these sex differences. Artifactual theories include sex differences in socioeconomic status as well as biases in the health care system. Biological theories include the role of genetics, hormones, and the immune system in health. A major set of contributors to sex differences in health are behavioral. Men smoke, drink alcohol, and use illicit drugs at higher rates than women; these sex differences are smaller among youth. However, men exercise more than women, and are somewhat less likely than women to be obese. These sex differences are influenced by race/ethnicity and LGBT status. Women and men are socialized in ways that influence their health. The male social role includes risk-taking behavior in the domains of work, driving, and leisure pursuits. The female social role involves greater concerns with health and caretaking behavior. Finally, agentic and communal traits have implications for health. The chapter concludes with a discussion of symptom perception and illness behavior.