Ecological systems and industrial systems have much in common. Both systems are characterized by flows of material and energy between components, both contain components that use energy to transform materials, and both contain energy-and material flow-regulating interactions such as competition and mutualism. These shared traits are reflected in the metaphor “an industrial system is an ecological system” that is central to industrial ecology. At the same time, critical differences exist between the two systems. Products, that is, goods and services exchanged for something of value, are characteristic of industrial systems, but relatively rare in ecological systems. This prevalence of products leads to a number of other interesting differences between the two systems, some of which might limit the value of ecological systems as prescriptive models for industrial systems.