In 2010, approximately nine million Americans self‐identified with more than one race on the U.S. Census – a 32 percent increase since 2000. In this paper, I review the growing body of research on this population, with a particular focus on identifying and describing factors important in shaping their racial identities. Factors explored include: social norms regarding racial classification, socioeconomic status, racial composition of one’s neighborhood and community, region, socialization by family, age, cohort, genealogical locus of multiracial ancestry, nativity, and phenotype. I discuss the broader implications of findings to‐date, with a particular focus on the ongoing scholarly discourse regarding the collection of race data in the United States.