It is no mystery that men and women are biologically and behaviorally different, but the way these differences impact mental health has often gone unrecognized. Sex and gender have both been increasingly identified as significant factors in disease prevalence, expression of symptoms, and responses to treatment. As such, it is critical that we understand the influence of sex differences and the consequences of adopting a "one size fits all" approach to health care. Unfortunately, this understanding has historically been difficult, if not impossible, to achieve because most of the knowledge we have in this area is based on research conducted exclusively in males. This disproportionate focus on male data is slowly beginning to change, but there are still those who doubt whether the presence/impact of sex differences is significant enough to warrant further investigation. In this course, we explore this debate, focusing on the neurobiology, methodology, significant findings, and future implications of research on sex differences.