The exchange of wastes, by-products, and energy among closely situated firms is one of the distinctive features of the applications of industrial ecological principles. This article examines the industrial district at Kalundborg, Denmark, often labeled as an “industrial ecosystem” or “industrial symbiosis” because of the many links among the firms. The forces that led to its evolution and to the interdependencies are described and analyzed. Key has been a sequence of independent, economically driven actions. Other potential forms of industrial linkages are critically reviewed in the light of the Kalundborg experience. The evolutionary pattern followed at Kalundborg may not be easily transferable to greenfield developments.