Complexity theory is destined to be the dominant scientific trend of the last decade. But what is it? It is the unifying theory which states that at the root of all complex systems lie a few simple rules. This revolutionary technique can explain any kind of complex system - multinational corporations, or mass extinctions, or ecosystems such as rainforests, or human consciousness. All are built on the same few rules. In Roger Lewin's immensely readable study the leading Complexity theorists tell the story in their own words. None of the sciences can ever be the same again.