The perception of the value and propriety of modern engineered systems is changing. In addition to their functional and extra-functional properties, nowadays’ systems are also evaluated by their sustainability properties. The next generation of systems will be characterized by an overall elevated sustainability — including their post-life, driven by efficient value retention mechanisms. Current systems engineering practices fall short to support these ambitions and need to be revised appropriately. In this paper, we introduce the concept of circular systems engineering, a novel paradigm for systems sustainability. After defining a conceptual reference framework to situate systems engineering practices within, we derive prerequisites for circular systems engineering. Finally, we outline the challenges and research opportunities associated with circular systems engineering.