In the previous chapter we used the concept of the health hierarchy to explore how health problems can be seen as the result of interactions between humans, and between humans and their environment. We saw that these elements and their interactions range from individuals' subjective experiences and cultural influences to the impact of global climate change. In this chapter we locate such interaction within the overarching theory of complexity. We summarise the new developments in complexity theory over the last decade that attempt to explain the full scope of these interactions. Complexity theory is now applied across a great variety of disciplines to more fully understand biophysical systems, human social systems and expressions of human culture and creativity. We believe that complexity theory is capable of providing greater insight into both the nature of disorder and the emergence of new forms of order within systems, but especially within health-related systems. We now describe the theoretical features of complexity theory in order to examine how it can be used to understand and inform problems of human health. In order to understand the impact and novelty of complexity theory we first examine earlier theories that attempted to describe complex systems and their characteristics.