Social movements are fuzzy phenomena with unclear boundaries. They may relatively quickly expand or shrink and change their structure and strategy. This makes it difﬁcult to identify, let alone empirically study social movements. Besides political parties and formal pressure groups, social movements can be seen as a major form of collective will formation and interest representation in modern societies. They usually signal structural strains in a given society and tend to challenge the established power holders by various means of protest. While many social movements failed to reach their aims, others had partial and few even had full success. In exceptional moments of history, social movements were able to bring about a new social and/or political order.
The term social movement refers to a speciﬁc kind of collective actor, namely a network of individuals, groups, and organizations who, based on a sense of collective identity, aim at changing society (or to resisting such a change) mainly by means of public and collective protest (della Porta & Diani, 1999; Rucht & Neidhardt, 2002).
Therefore, first, in structural terms, a social movement is a loosely coupled conglomerate of different components. Contrary to speciﬁc social movement organizations which may have formal statutes, a rigid division of labor, and a clearly deﬁned membership, social movements are networks
which, by deﬁnition, are mainly horizontally and informally coordinated. Such a network can include one or several core groups. Yet there is no central body which would be able to control and dominate the movement as a whole. In terms of degrees of involvement of individuals, a social movement can be conceived as a set of concentric circles ranging from highly committed activists in the core section to the periphery of mere sympathizers who only occasionally express their support to the movement.
Second, in terms of its aims, a social movement seeks to fundamentally change society (or to resist such changes). Ultimately, it targets the prevailing social order including its power structure and basic values. Because a power structure also crystallizes in political institutions and processes, a social movement is necessarily involved in political struggles. Still, it makes sense to distinguish a broadly conceived social movement from more narrowly deﬁned collective actors who primarily focus on political, cultural, or religious matters and could be termed as political, cultural, or religious movements, respectively.
Third, regarding its means of cohesion, a social movement rests on a sense of collective identity, i.e., a we feeling based on shared worldviews, values, claims, and practices that allow to separate the movement as an (imagined) entity from mere bystanders and, above all, from the movement’s opponents. Collective identity is not a pre-existing and stable property based on a self deﬁnition only. Rather, it emerges and develops in interaction with the movement’s environment which, to some extent, also shapes the movement’s identity.
Fourth, social movements strongly rely on the commitment of their constituents who, from time to time, are mobilized in protest activities. These are the major tool to make the movement seen and heard beyond its own ranks. It depends on structural and situational factors whether or not a social movement mainly engages in direct confrontation with an adversary or, especially in democratic systems, tries to inﬂuence its opponents indirectly by attracting public attention and, hopefully, support. The strategies and, even more so, the speciﬁc forms of protest may vary greatly within and across movements. Some movements tend towards a reformist, others towards a revolutionary strategy. But as many historical examples demonstrate, the same movement may include a moderate and a radical tendency. Accordingly, the speciﬁc protest actions range from friendly (e.g., collection of signatures) to confrontational (e.g., blockades) to violent means (physical aggression and destruction). Whether or not terrorism should be included in the concept of social movements is disputed. Based on the broad deﬁnition proposed here, there would be no reason to set terrorism completely apart, though very few movements systematically resort to extreme violence.Several perspectives used for understanding social movement include classical approach, rational choice, resource mobilization, new social movement, and political opportunity structure.
A social movement, although it may have secondary aims to promote the development of a subculture, is inherently and fundamentally a political phenomenon since its target is the state. This contention is more clearly demonstrated if we start with Max Weber's (1958) definition of the state as ``a human community that (successfully) claims the monopoly of the legitimate use of physical force within a given territory.'' Since the state holds this legal use of coercive power, it reserves a position as the primary distributor of rights and resources. A social movement, in its attempt to acquire these resources to promote structural change, must view the state as both an objective and an antagonist. Thus, a movement requires the resources of the state it abhors in order to alter that state. An antagonistic relationship exists between the state and the social movement, or between those individuals who hold political power and those who are effectively disenfranchized. Hence, social movements embody sustained collective challenges against political elites and authorities led by people with a common purpose and who lack regular access to existing political institutions.
Classical and rational choice theories are micro-level analyses, which mean that they focus on individual motivation to participate in a social movement. Why does an individual choose to participate? As such, these theories concentrate on the existence of individual grievances. Given their base assumption that democratic society is essentially pluralistic or that all voices are heard, classical and rational choice theories perceive collective behavior as fundamentally irrational and indicative of some kind of psychological imbalance; collective action thereby falls outside the realm of legitimate politics.
Encountering the increase of collective behavior in the 1960s, the evidence that participants in such action did not suffer from some psychological dysfunction, and increasing belief that American politics was elitist rather than pluralistic, students of social movements developed a new theory, resource mobilization, to understand the formation of such activity. A caveat of an elite-managerialist theory of democracy is that grievances perpetually exist: ``there is always enough discontent in any society to supply grass-roots support for a movement if a movement is organized and has at its disposal the power and resources of some established elite group;” the centrality of organizational needs and resources, e.g., money, labor, office space, is established as a primary factor in movement development. The research question is altered from why individuals participate to how such participation is possible. In short, resource mobilization is a mesolevel analysis focusing on organizational requirements of social movements.
Classical theory, rational choice, and resource mobilization cluster about the stereotypical American school, which emphasizes individual or collective agency, as opposed to the more Marxist perspective, which places stress on structure. In none of the three theories discussed above is any reference made to traditionally Marxist notions of solidarity, collective identity, or collective (class) consciousness.
The aim of new social movement theory is to explain the existence of post-1960 movements which do not turn on class consciousness, but on identity – a phenomenon of advanced industrial and/or late-capitalist society – while simultaneously reviving a traditional Marxist interpretation through the incorporation of the participants' collective identity. New social movement theory is a collection of neo-Marxist theories operating at the intersection of micro- and macro-analytic spheres; it seeks to understand why individuals participate in collective behavior via reference to grievance articulation while also claiming the structuralist view that identity is shaped by the overarching circumstances and dynamics of advanced industrial society.
In response to new social movement theory, the American school was forced to focus more fully on macro-level analysis, that is, the interaction between the state and the social movement without relying on a Marxist interpretation. Political opportunity structure (POS) filled this theoretical gap. Political opportunity structures refer not to necessarily permanent nor formal configurations of political institutions and historical precedents for social mobilization which protesters can exploit to promote collective action. In essence, at a macro-level then, social movement formation boils down to a question of timing; as sociologist Sidney Tarrow notes, ``People join in social movements in response to political opportunities and then, through collective action, create new ones. As a result, the `when' of social movement mobilization – when political opportunities are opening up ± goes a long way towards explaining its `why.'' Therefore, political opportunity structure is a macro-analysis that evaluates how different governing structures affect mobilization by providing possible institutional opportunities such as electoral realignments; hence, POS is useful when comparing and contrasting similar movements in different countries.
All theories are relatively incapable of accurately depicting and evaluating historical circumstance; however, some are better able to minimize the distorting lens. The aim, when understanding social movements, is not to develop a totalizing theory that accounts for every potential variable, but to derive a model which can answer the foundational questions of any given analysis. In this case: why people participate (micro-level), how they (afford to) participate (meso-level), and when they participate (macro-level). Each of these questions underlies a specific focus whether it be oriented toward individual motivation, organizational resources, or the external/institutional environment.The political process model (PPM) accomplishes this by successfully navigating the junctures of the micro-, meso-, and macroanalytical levels to isolate three crucial factors in the development of collective insurgency: changing opportunity, pre-existing organizational strength, and cognitive liberation coupled with collective identity formation. PPM is essentially made up of two distinct but related dynamics: movement formation and movement maintenance. The first part is represented by the tri-factor interaction of a changing opportunity structure, organizations to seize control of this opportunity, and the psychological alteration of a minority group from an isolated victimized perspective to a sense of collective empowerment. The second part of the model refers to the actions of the organizations that make up the movement. These networks are sometimes called pressure groups, special interests, or interest groups. Unlike a movement, interest groups are characterized by defined membership, permanent staff, and fiscal responsibilities. More importantly, whereas a movement is sometimes characterized as acting outside of the political system, interest groups navigate from within it. They interact with the political institutions – the legislative-executive system, the judiciary structure, and the political party dynamics of a state - in an attempt to reform the ruling ideology.
 For more detailed synopsis of the classical approach see Doug McAdam, ``The Classical Model of Social Movements Examined'' in Social Movements: Perspectives and Issues, eds. Steven M. Buechler and F. Kurt Cylke, Jr. (Mountain View, California: Mayfield Publishing Company, 1997) 135±48; William Kornhouser, ``The Politics of Mass Society'' in Social Movements: Perspectives and Issues, eds. Steven M. Buechler and F. Kurt Cylke, Jr. (Mountain View, California: Mayfield Publishing Company, 1997), 91±7; Mancur Olson, The Logic of Collective Action (Cambridge, Mass.: Harvard University Press, 1965); John Elster, Nuts and Bolts for the Social Sciences (New York: Cambridge University Press, 1989), 22±42, 124±73.
 Hank Johnston, Enrique Larana, and Joseph Gusfield, ``Identities, Grievances, and New Social Movements'' in Social Movements: Perspectives and Issues, eds. Steven M. Buechler and F. Kurt Cylke, Jr. (Mountain View, California: May®eld Publishing Company, 1997), 274±94. Alberto Melucci, ``The New Social Movements: A Theoretical Approach,'' Social Science Information 19 (2) 1980, 119±26.
 Herbert Kitschelt, ``Political Opportunity Structures and Political Protest: Anti-Nuclear Movements in Four Democracies'' in American Society and Politics, eds. Theda Skocpol and John L. Campbell (New York: McGraw Hill, Inc., 1995), 321. Tarrow, Power in Movement, 85.
 Ronald J. Hrebenar, Interest Group Politics in America (Armonk, NY: M. E. Sharpe, 1997)