The idea of complexity states that most things tend to organize themselves into recurring patterns, even when these patterns are not immediately visible to an external observer. The general name for the scientific field concerned with the behaviour over time of a dynamic system is "complexity theory". The dynamic systems - systems capable of changing over time - are the focus of this approach, and its concern is with the predictability of their behaviour. The systems of interest to the complexity theory, under certain conditions, perform in regular, predictable ways; under other conditions they exhibit behaviour in which regularity and predictability is lost. The concepts of stable and unstable behaviour are part of the traditional repertoire of physical science. What is novel is the concept of something in between - chaotic behaviour. For chaos here we refer to systems which display behaviour that, though it has certain regularities, defies prediction. How does the order emerge from the chaos? How can we predict the behaviour of a chaotic system? Over the last 30 years and more, trying to identify the hidden patterns behind chaotic behaviour became the focus of attention in a number of scientific disciplines. These range as widely as astronomy, chemistry, evolutionary biology, geology and psychology.