This paper reviews the role of conventional waste-to-energy, i.e. incineration of (mainly) municipal solid waste with energy recovery, in the circular economy. It shows that, although waste-to-energy figures on a lower level in the European waste hierarchy than recycling, it plays, from an overall sustainability point of view, an essential, complementary and facilitating role within the circular economy. First of all, waste-to-energy combusts (or should combust) only waste that is non-recyclable for economic, technical or environmental reasons. This way waste-to-energy is compatible with recycling and only competes with landfill, which is lower in the waste hierarchy. Furthermore, waste-to-energy keeps material cycles, and ultimately the environment and humans largely free from toxic substances. Finally, waste-to-energy allows recovery of both energy and materials from non-recyclable waste and hence contributes to keeping materials in circulation. These arguments are elaborated and illustrated with many examples. This paper also points out the pitfalls of a circular economy if it merely focuses on material cycles, disregarding economic, environmental, social and health aspects of sustainability.