This article considers the place of knowledge in developing a socially just curriculum. It pursues the unusual route of a critique of Social Realism, a small but influential tendency in curriculum studies which claims that knowledge has been squeezed out by recent curriculum reforms and that there has been a descent into relativism. This paper shares the Social Realist view that 'powerful knowledge' is needed, and particularly by disadvantaged or marginalised young people. However, it critiques Social Realism's limited definition of 'powerful knowledge,' arguing that for knowledge to be truly powerful, it must open up issues of power and inequality. It contests the Social Realist argument that critical pedagogy which begins from a subaltern stance is intrinsically relativist, arguing instead that alternative perspectives can help uncover concealed truths and break through hegemonic paradigms and ideologies. It argues that this is entirely compatible with a Critical Realist epistemology. Furthermore, the paper presents reasons why a socially just curriculum needs to draw upon the vernacular knowledge of marginalised groups as well as the canonical knowledge of academic disciplines to produce truly powerful knowledge and a social justice curriculum.