Historically females of all ages have been underrepresented in clinical research. Reasons for this exclusion are multifactorial and may possibly have their origins in regulation that forbid the participation of females of childbearing potential in the earliest phase clinical studies that support drug approval. Decades of female underrepresentation in clinical studies has resulted in inequality in the understanding, diagnosis, and treatment of disease between the sexes. Adequate numbers of both sexes is one approach which is likely to present overwhelming financial constraints. Advances in study design, statistical methodologies, and the promise of evolving technologies will lead to new tools that can foster a better understanding of the biology that governs sex and gender differences.