This article outlines a theory of what affects the degree and nature of women’s participation in the armed forces throughout history and across nations. Examining national security situations, military technology, military accession policies, demographic patterns, cultural values regarding gender, and structural patterns of gender roles, the article proposes a systematic theory of the conditions under which women’s military roles expand and contract. The theory is then applied to analyze women’s likely future role in armed forces. The military’s need for personnel has been the driving force behind the expansion of women’s military roles through and across nations, but cultural values supporting gender equality also contribute and seem likely to have increased influence in the future.