Sex is a fundamentally important biological variable. Recent years have seen significant progress in the integration of sex in many aspects of basic and clinical research, including analyses of sex differences in brain function. Significant advances in the technology available for studying the endocrine and nervous systems are now coupled with a more sophisticated awareness of the interconnections of these two communication systems of the body. A thorough understanding of the current knowledge, conceptual approaches, methodological capabilities, and challenges is a prerequisite to continued