Lewin (1951) recognized that it “is usually easier to change individuals formed into a group than to change any one of them separately” (p. 228). More than sixty-five years later, social identity theory (SIT; Tajfel & Turner, 1979) and self‐categorization theory (SCT; Turner et al., 1987; collectively referred to as the social identity perspective/approach) offer many insights into why and how this is the case. At the heart of the social identity perspective is a comprehensive and systematic theory of the “group,” which generated a new view of the self-process where humans are both individuals and group members with both personal (“I”) and social (“we”) identities. Importantly, social and personal identities can change and, in turn, so too can behavior. An important part of the behavior change “puzzle”, often overlooked by researchers, policy makers, and practitioners, is that it is necessary to engage not only the “I” or “me” but also the “we.” This chapter outlines the potential of social identity processes, including in-group norms and social influence, in advancing understanding of behavior change. Taken together, research and practice applying the social identity approach to behavior change demonstrate considerable promise in promoting change in group contexts and for multiple behaviors in multiple domains such as work, education, and community settings.