Extremist groups rely upon women to gain strategic advantage, recruiting them as facilitators and martyrs while also benefiting from their subjugation. Yet U.S. policymakers overlook the roles that women play in violent extremism—including as perpetrators, mitigators, and victims—and rarely enlist their participation in efforts to combat radicalization. This omission puts the United States at a disadvantage in its efforts to prevent terrorism globally and within its borders. The number of women implicated in terrorism-related crimes is growing. The failure of counterterrorist efforts to understand the ways in which women radicalize, support, and perpetrate violence cedes the benefit of their involvement to extremist groups. Omitting women from terrorism prevention efforts also forfeits their potential contributions as mitigators of extremism. Women are well positioned to detect early signs of radicalization, because fundamentalists often target women’s rights first. As security officials, women provide insights and information that can be mission critical in keeping the peace. And because of their distinctive access and influence, women are crucial antiterrorism messengers in schools, religious institutions, social environments, and local government. Overlooking the contributions women can make to prevent extremism renders the United States less secure. Many extremist groups promote an ideology that classifies women as second-class citizens and offers strategic and financial benefits through women’s subjugation. Suppressing women’s rights also allows extremists to control reproduction and harness female labor. U.S. government policy and programs continue to underestimate the important roles women can play as perpetrators, mitigators, or targets of violent extremism. To help prevent and reduce terrorism, the Donald J. Trump administration should produce a National Intelligence Estimate to analyze the ways in which women provide material support to extremist groups; increase resources to facilitate women’s involvement in terrorism prevention efforts; and improve the recruitment, retention, and advancement of women across the security sector to bolster the capacity of forces to mitigate potential terrorist threats. These steps will help the United States and its allies respond effectively to the security threat posed by violent extremism and advance U.S. peace and stability.