In recent decades, scholars have documented remarkable changes in women's political representation around the world. At present, however, researchers are ill-equipped to explain these changes in women's representation in politics. In this article, we introduce a broad theoretical framework for reorienting existing explanations of women's formal political representation to account for change over time. We conceptualize stasis and growth in women's political representation as the balance between forces of resistance and forces for change. We also classify forces by timing, distinguishing among those that are continuous, are episodic, and have originated from critical periods. Using data on women in national legislatures from 1945 to 2003, we find that longitudinal forces combine to produce four common trajectories of women's political representation across time. We also discuss how various approaches, including event history analysis, latent growth curve models, and the focused ethnographic revisit, are well suited for modeling the different types of forces that combine to produce these trajectories. Overall, we argue that implementing a longitudinal framework for understanding women's political representation has the power both to alter the way we think about established findings and to suggest new theories for empirical evaluation.