The exposition, analysis and critique of critical realism and its implications for curriculum are considered in this chapter and the next. This chapter establishes the basis for knowledge, while the next considers the relationship between knowledge and the objects that the knowledge is about, how knowledge changes, the relationship between the disciplines and how students are inducted into the disciplinary structures of knowledge. Thus, the focus of this chapter is ontological because it is concerned with the nature of what exists. It asks the critical realist question: what must the world be like if we are to have knowledge of it? The focus of the next chapter is epistemological because it asks: given that the world is real and exists independently of our conceptions of it, how do we gain knowledge of it? The way we understand the world to be structured and constituted (the ontological) consequently sets boundaries around the way we gain knowledge of it (the epistemological).