The chapter discusses the phenomenon of organizational similarity, attributing it to the shift from competitive markets to the state and professional categories. It describes three isomorphic processes—coercive, mimetic, and normative—that lead to increased similarity among organizations. The authors define hypotheses about the impact of resource centralization, structuration, ambiguous goals, and technical uncertainty on isomorphic change. They also explore implications for organizational theory and social change. The chapter highlights how the rationalization and bureaucratization mechanisms have evolved, moving from market competition to state and professional control. It argues that highly structured organizational fields drive structural changes, leading to homogeneity in structure, culture, and outcomes. The authors identify three institutional mechanisms of isomorphic change: coercive, mimetic, and normative, each with distinct antecedents and outcomes. They provide hypotheses predicting the conditions under which these mechanisms operate and suggest that the increasing professionalization and structuration of fields contribute to isomorphic changes. The chapter concludes by discussing the implications of these findings for understanding organizational diversity and social change.The chapter discusses the phenomenon of organizational similarity, attributing it to the shift from competitive markets to the state and professional categories. It describes three isomorphic processes—coercive, mimetic, and normative—that lead to increased similarity among organizations. The authors define hypotheses about the impact of resource centralization, structuration, ambiguous goals, and technical uncertainty on isomorphic change. They also explore implications for organizational theory and social change. The chapter highlights how the rationalization and bureaucratization mechanisms have evolved, moving from market competition to state and professional control. It argues that highly structured organizational fields drive structural changes, leading to homogeneity in structure, culture, and outcomes. The authors identify three institutional mechanisms of isomorphic change: coercive, mimetic, and normative, each with distinct antecedents and outcomes. They provide hypotheses predicting the conditions under which these mechanisms operate and suggest that the increasing professionalization and structuration of fields contribute to isomorphic changes. The chapter concludes by discussing the implications of these findings for understanding organizational diversity and social change.