This article traces the evolution of the eco-city as a concept and an urban planning model over the last 40 years, outlining the various definitions, applications and critiques of the term historically and today. What distinguishes the eco-city from work on sustainable urbanism more broadly, the article argues, is its attempt to create a comprehensive and transferable model of sustainable urban development. Over the years, the eco-city, in both theory and application, has evolved along with broader trends in environmental thought. The idea of the eco-city originally emerged out of counterculture movements of the 1960s and 1970s as an approach to urban development that would respect environmental limits. Contemporary eco-city projects attempt to transcend these limits and are often driven as much by economic objectives as environmental ones. Although many eco-city projects market themselves as models for future urban development, the article argues that they are better seen as sites of experimentation and innovation, helping drive broader socio-technical transitions. The article concludes that the ability of eco-cities to achieve their utopian ambitions may be limited by the realities of operating within a profit-driven, entrepreneurial planning environment. However, they can still play a valuable role, providing a place to test new ideas and an ideal to aspire to.