Both scholars and public officials have advocated for the incorporation of measures of sexual and gender identity on nationally representative surveys. Recommendations such as those put forth by a 2022 panel of the National Academies of Sciences, Engineering, and Medicine typically caution that identity is fluid and most sexual orientation measures only capture identification at a given point in time. Nonetheless, demographers typically rely in their analyses on cross-sectional data that treats sexual identity as fixed. In this chapter, we examine what is known about sexual identity fluidity, beginning with the prevalence of fluidity within various subpopulations. We then discuss individual characteristics that are associated with fluidity, most notably gender and sexual identity, as well as contextual characteristics such as region and sociopolitical context. Finally, we review the literature on the relationship between sexual fluidity and demographic outcomes, including physical and mental health, earnings, and experiences of victimization and discrimination. We conclude by offering recommendations for future data collection and research related to sexual identity fluidity.