Cities are centers of human and economic activity but also of resource use and waste. This gives cities both a critical and a promising role to support the transition to a circular economy by keeping incoming products and resources in the loop. A circular city is a place, where people share the resources they have, facilitated through business models that avoid losses and build on maximizing resource productivity. This requires a redesign of biological and technical material cycles in a way that their value can be maintained at the highest possible level for as long as possible. This chapter addresses questions such as: What could a circular city look like? What does this mean in practice? Where can we already see a transition? And what can urban policymakers and other actors do to realize circular cities? In this chapter, we will explore the resource streams flowing into cities, their main challenges and opportunities, solutions to close biological and technical cycles, the ways to measure progress, the actors and their roles within the circular city fabrics, and finally, a case comparison of circular city practices.