This chapter examines critical theory as a way of explaining, contesting and acting upon social injustice. Critical theories, of which there are many, have been a significant source of explanatory power and motivating action for social work. As will be shown, there are a number of different generations and formulations of critical theory. What links these different theories is that they combine explanations of social reality with practical aims for addressing social injustices. The chapter explains what makes critical social science critical, and then outlines Marxist, feminist and postcolonial theory by explaining each position in relation to a theory of false consciousness, theory of crisis, theory of education and a theory of transformative action.