Although myriad sex diferences have been found in the brain, the functional consequences of these diferences are understood in only a few cases. This chapter presents new insights derived from broad-scale genomic approaches to studies that focus on sex diferences in the expression of single genes. It illustrates the pervasiveness and interconnectedness of sex diferences in the brain. Consideration is also given to the development and function of sex diferences in brain function. A case will be made that sex diferences in the brain can only be fully understood against a whole-body perspective. Although it is too early for a full-scale translation of the observed sex diferences in structure into sex diferences in overt functions, current knowledge underscores the need to consider sex as a biological variable. Doing this will improve scientifc discourse, and considering sex diferences will refne medical practice.