Already in agricultural societies issues like the efficient use of natural resources, recycling of goods and materials, and minimization of food waste were key principles. Current circular economy initiatives are building firmly on these same principles. Often circular economy actions concentrate on material efficiency ignoring other possible flows. This paper conceptualizes circular cities by reviewing their constraints, characteristics and connections. Even a proper definition for a “circular city” is missing. After analyzing circular economy (CE) challenges and opportunities in cities, the paper concentrates on characteristics of circular city concepts, namely definition, transition process, target setting and circularity indicators. These are the core issues a city has to define when targeting circularity. In addition, the paper addresses city streams, their interlinkages and boundaries which a city has to analyze when deciding about the concrete actions and means. Adequate tools are needed in this phase to analyze circular cities. Potential Nexus and expert tools were identified in the paper. The paper ends up with a synthesis of steps cities need to take when targeting circularity.