Some of the ideas about “chaos” emerging in various fields of the natural, social, and applied sciences have major implications for planning. This article looks at chaos (defined as “order without predictability”) and shows how anyone with an ordinary microcomputer can investigate the nature and development of chaos. Chaos theory promises to have profound implications for what planners do and how they do it. It suggests that the world may be both easier and more difficult to understand than we tend to believe, that noisy and untidy cities may not be as dysfunctional as we often assume, and that the need for planning that is incremental and adaptive in nature may be more urgent than we tend to think.