The theory of emergence matters because it provides the essential foundation for understanding how causal forces operate in the world. This chapter is dedicated to explaining the relationship between emergence and cause in general, so that the rest of this book can go on to show how this underpins the ontology of the social world. In particular, the first half of this chapter connects up the relational account of emergence given in chapter 2 to the critical realist model of cause developed in the early work of Roy Bhaskar. The combination of these two, it argues, provides a much stronger understanding of cause than the influential covering law model of cause arising from the work of David Hume and Carl Hempel. The second half of the chapter aims to show how the relational conception of emergence enables us to overcome two common challenges to emergentism. The first is the reductionist claim that the causal impact of emergent higher-level entities can be explained purely in terms of the impacts of their parts. The second is the argument that emergentist theories imply, but cannot explain, the phenomenon of downward causation – a causal impact of wholes on their own parts, such as the effects that social structures may have upon the individuals that compose them (to be illustrated in chapters 6 and 7).