Unlike their more common linear counterparts, cyclic polymers have a ring-like structure and a lack of chain ends. Because of their topology, cyclic polymers exhibit a unique set of properties when compared with linear or branched macromolecules. For example, cyclic homopolymers exhibit a reduced hydrodynamic volume and a slower degradation profile compared with their linear analogues. Cyclic block copolymers self-assemble into compact nanostructures, as illustrated by their reduced domain spacing when cast into thin films and their reduced micellar size in solution. Although methods for preparing well-defined cyclic polymers have only been available since 1980, the extensive utilization of the cyclic topology in nature highlights the vital role that a cyclic architecture can play in imparting valuable physical properties, such as increased chemical stability or propensity towards self-assembly. This Review describes the major developments in the synthesis of cyclic polymers and provides an overview of their fundamental physical properties. In this context, preliminary studies exploring potential applications will be critically assessed and the remaining challenges for the field delineated.