During the past 20 years, there has been a dramatic shift in scientific, ethical, and legal perspectives regarding the inclusion of women in clinical trials conducted in the United States. A historical overview of why women previously were excluded from clinical trials is presented, and the reasons for current policy changes are discussed. The clinical necessity for testing drugs in specific populations (e.g., women) to ensure appropriate dose regimens and to minimize the likelihood of adverse effects is discussed. The difficult issue of prescribing drugs for pregnant women is highlighted.