With this Dear Colleague Letter (DCL), in response to the White House Initiative on Women's Health Research, the U.S. National Science Foundation (NSF) encourages the submission of research and education proposals related to women's health. Despite making up more than half of the population, women are historically understudied and underrepresented in health research. The historical exclusion of women from scientific and biomedical research studies, combined with the undervaluation of research that advances knowledge on conditions that uniquely, differentially, or disproportionately affect women, has resulted in significant knowledge and health gaps. Addressing these research gaps will ultimately advance the health, prosperity, and welfare of all.