This chapter presents the major theories and approaches in social movement research, highlighting four constitutive elements. Next, in a more descriptive approach, it outlines some general patterns and profiles of contemporary social movements, stressing the need to interpret social movements in relation to their key reference groups of which, besides movements’ opponents, allies, third parties, and bystanders play a crucial role. Another section is devoted to comparative aspects and findings in social movement studies, including the more recent trend towards the transnationalization of some movements. Finally, a typology of the basic functions and effects of social movements is presented. The general theme is that the structural context, the shared perceptions and experiences, as well as the dynamic interactions between social movements and their reference groups are crucial for a better understanding of the functions and effects of social movements as potential creators of social change.