Complex systems are all around us including, for example, biological cells, bee colonies, the brain, climate, telecommunication infrastructures, the stock market, and the economy. Typically, they consist of many distinct but interacting elements, and they may be characterised by states of the elements. But the states change as a result of the interactions, and the interactions themselves change corresponding to the states of the system. It is this chicken-and-egg situation that gives rise to the complex behaviour and the well-known difficulties in understanding it, let alone predicting it. In particular, it gives rise to features such as emergent behaviour that could not have been anticipated from a knowledge of the elements, however detailed.