In this chapter, we provide a working definition of legitimacy. We then articulate an analysis of the social process of legitimacy, focusing on how social objects, such as authorities and practices, gain legitimacy. We also examine the bases (i.e., instrumental, relational, and moral) of the legitimation process and present an example of how leaders acquire legitimacy in the workplace. We then discuss the stability of legitimacy and address questions of how social objects/activities maintain legitimacy and why practices are difficult to change once legitimated. We then articulate consequences of legitimacy for interaction and outcomes in groups and organizations, noting when legitimacy is a good or bad thing. Near the end of the chapter, we present recent multilevel theories of the legitimacy process that focus on how legitimacy (specifically propriety) can become a force of change within organizations, that is, how procedures, policies, or practices may become delegitimated and new ones adopted. We also discuss recent work by sociological social psychologists that comment on this multilevel theory. In doing so, we build on previous work that addresses the relationship between individual and collective bases of legitimacy. Throughout the chapter, we also illuminate how legitimacy processes at the micro level shape and are shaped by the meso and macro levels of social reality. We conclude with suggested future directions.