Although Roy Bhaskar’s ontology in A Realist Theory of Science (Bhaskar 1978) is explicitly stratified into a hierarchy of levels, he makes relatively little attempt to examine the basis of this stratification, or its implications for his three domains of the empirical, the actual and the real (Collier 1994: 130).2 This chapter attempts to remedy that deficiency, by investigating the nature of a stratified reality based on emergence, and considering how this impacts our understanding of experiences, events, entities, and causes. Bhaskar uses ‘stratification’ to indicate two quite different ontological schemes: the stratification of the world into emergent explanatory levels, which I shall call level stratification, and the division of ontology into domains, which I shall call domain stratification. It is the relation between these two different schemes that is the central theme of this chapter.