This book surveys the current research on CO 2  conversion processes and shows that these can close the carbon cycle as part of a circular economy. The technical and economic feasibility of these processes are examined together and current scientific challenges are signposted, which will guide future R&D. Technology sustainability is key for meeting and keeping decarbonization goals in the long term. However, considering economic and environmental sustainability individually is not enough. An integral view of sustainability that incorporates an energy term in the equation is needed. This book brings this concept to the fore.