In this article, I review the social science literature on racial fluidity, the idea that race is flexible and impermanent. I trace the ongoing evolution of racial classifications and boundaries in the United States and Latin America, two regions that share a history of European colonization, slavery, and high levels of race mixing but that have espoused very different racial ideologies. Traditionally, for many groups in the United States, race was seen as unchangeable and determined by ancestry; in contrast, parts of Latin America have lacked strict classification rules and embraced race mixing. However, recent research has shown that race in the United States can change across time and context, particularly for populations socially defined as more ambiguous, while some Latin American racial boundaries are becoming more stringent. I argue that the fluidity of race has redefined our understanding of racial identities, and propose several directions for future political science scholarship that bridges disciplines and methodological approaches.