Although the concept of the eco-city has passed into mainstream policy-making and urban planning internationally, it has no agreed definition, either as a set of normative principles or practical developmental goals. This chapter therefore introduces the subject matter by acknowledging its multiplicity. First, it reviews the merits of a series of different definitional approaches: the eco-city as promotional label; as a certain type of megaproject; as a broader analytical category; as a set of normative principles; as a provocative oxymoron; and as an experimental process. It then outlines some of the key critical perspectives that have been developed on eco-city development in practice. Despite these criticisms, the chapter concludes by inviting optimism about the potential for the eco-city, as a variegated process of experimentation and reflection, to provide clues about the nature of ‘cityness’ and the role of citizens in a more sustainable future.