[Excerpt] Resource mobilization, a dominant theoretical approach to the study of social movements for many decades, points to social movement organizations (SMOs) as a focal point for efforts to understand the variations in both the impact and fate of social movements. SMOs, like other types of political organizations, are expected to represent members’ common preferences for some specified social change, acting to bring about such change through influence on formal political decision-making, or on general behaviors of the members of a polity, or on both. In this context, the classic analysis offered by Robert Michels ( 1962) of typical evolutionary processes in the governance of political organizations, and the impact of such processes on organizations’ goals, is very relevant to scholars of social movements. Early studies of social movements often drew heavily on Michels’ work, documenting and fleshing out the nature of the evolutionary processes he posited, and the transformational consequences for social movements. Concern with movement transformation has been less dominant in contemporary work, despite a lack of evidence that such processes are any less operative in current movement organizations. Below, the key processes involved in what Michels’ referred to as the “iron law of oligarchy” are sketched, followed by a brief discussion of some of the implications of this analysis for social movement researchers.