AbstractStandard demographic research has typically constrained the existence of identity fluidity, assuming that demographic categories such as race or ethnicity and sex or gender should be static across the life course and measuring them as such. However, recent research and changes in data collection practices highlight the limitations of this approach by demonstrating fluidity in both racial and gender identities that rivals levels of fluidity in other identity categories, such as sexual orientation, that are more commonly seen as changeable over time. This review examines what is known about current levels of fluidity in gender, sexual orientation, and racial identities as well as known correlates and consequences for research on inequality, based primarily but not exclusively on research conducted in the United States. The implications of fluid identities for data collection and analysis, as well as prospects for future levels of fluidity, are also discussed.