Urban sociology and urban studies increasingly employ the idea of sustainability to explain, analyze, and critique city redevelopment. While the ambiguous and oxymoronic nature of sustainability goals has been extensively covered in the past, the current resurgence and popularity of the term “sustainability,“ especially under the aegis of “urban sustainability“ or “green“ cities, requires us to rethink the usefulness of sustainability as a concept for understanding and evaluating urban redevelopment. Confronting this challenge, this article reviews three of the most common theoretical approaches to sustainability, problematizes those approaches in the context of North American cities, and then provides a working definition of urban sustainability. Finally, the article recommends four plausible research hypotheses to guide future research on urban sustainability.