This chapter clarifies the concept of revolution, to prepare the way for later chapters that present a theory of revolution as the most dramatic form of resistance to hierarchy. Revolution is distinguished from coups, secessions, and more limited rejections of authority. A distinction is also drawn between political and social revolutions. The chapter then goes on to provide a comprehensive account of what ideologies are and of the explanatory power of appeals to ideology in theories of social change, with special emphasis on the role of ideologies in revolutions. Ideologies are defined as coherent but not necessarily consistent sets of beliefs, attitudes, and belief-management processes that provide individuals with shared evaluative map of the social world. Next, the chapter explains the ways in which ideologies can, depending on the circumstances and the nature of the ideology, either contribute to social change or help maintain the status quo. This chapter emphasizes the fact that the motivating power of ideologies is due, in large part, to their including moral norms and commitments. The chapter also explains that it is by virtue of these moral elements that ideologies can either enable or inhibit collective action aimed either at changing the status quo or sustaining it.