Cities are often seen as symbols of order: the existence of city walls, fortified gates, palaces, temples, roads, pavements, highways, public institutions, city centers and residential areas is interpreted as indicating the existence of a central authority that plans and controls the city. On the other hand, the very same cities are also seen as symbols of chaos, disorder and spontaneous growth. The little winding streets and alleys, the mixture of physical structures, styles and human activities have often given the impression that cities, like forests, and other natural entities are organic structures - strange, natural artefacts. Tokyo is a good example for this dual nature of cities. When you first encounter it, you get the impression of chaos: old buildings with one or two storeys next to 30-, 40- or 50-storey skyscrapers; pedestrians, cars, trains moving in all directions, each with its own trajectory and so on. But then you realise that this seemingly chaotic structure provides a context for perfectly ordered human activities: trains leave and arrive as timetabled, their doors open at the exact points that are marked with yellow lines on the platforms, every morning after midnight fishermen bring their catch to Tokyo's big fish market, auctions are held, and by six o'clock in the morning this huge amount of sea food has already been distributed among thousands of restaurants all over the city. And if you look deeper you learn that the chaotic face of Tokyo is the pre-condition for its ordered and organised life. Complexity theory or self-organisation theory are umbrella terms for a set of theories that study the interplay between chaos and order. Originating in the sciences, these theories have been applied to the study of cities in the last three decades. They show that as in natural systems, in the artificial systems that we call cities, chaos and order do not stand in opposition to each other. Rather, they coexist in an ongoing interplay of circular causality: chaos is the precondition for new urban orders to emerge and then to reproduce themselves, whereas order and organisation set the boundaries within which chaotic structures and behaviours can take place.