This chapter examines the critical role of renewable energy and energy efficiency in circular economy liveable cities. As cities account for the majority of global energy use, transitioning to renewable energy and improving energy efficiency are essential for achieving climate goals and sustainable urban development. The chapter emphasises how circular economy principles can enhance energy systems by promoting the use of renewable energy, reducing resource consumption, and minimising waste. Areas of focus include the integration of renewable energy sources, such as solar, wind, and waste-to-energy systems, into urban infrastructure. The chapter discusses innovative technologies like smart grids, energy storage solutions, and shared mobility systems that can optimise energy use and reduce environmental impacts. It explores energy-efficient practices in the built environment, such as green building design, retrofitting, and modular construction, which help minimise cities’ energy footprint. The chapter highlights case studies from European cities that have successfully implemented circular energy systems, demonstrating the effectiveness of combining renewable energy with circular economy practices. It concludes by addressing the challenges and opportunities for cities to foster sustainable energy transitions, emphasising the importance of policy support, public–private partnerships, and community engagement in achieving long-term energy efficiency and renewable energy goals.