Social identity is a central concept in understanding how race and racism are internalized, the relationship of individual and group identities, and for understanding how identities influence organizations, communities, nations and are an essential part of the clinical process. This chapter reviews and critiques theories of identity, social identity, and racial and ethnic identity. We then present our model of multidimensional identity development, which is not linear and does not place value or judgment on how people resolve and integrate their identities. We consider the specific dynamics and challenges for multiracial people. We conclude the chapter with a description of how identity shapes intergroup dynamics and consider the implications of this when working towards racial justice.