Globally, more than 50% of all people are living in cities today. Enhancing sustainability and efficiency of urban energy systems is thus of high priority for global sustainable development. The European research project PLEEC (Planning for Energy Efficient Cities) focuses on technological, innovative, behavioural and structural capacities of European medium-sized cities in their transition towards Energy Smart Cities. The variation of strengths and weaknesses of cities’ capabilities as well as practices and tools for enhancing energy efficient performance of urban energy systems were at the centre of the project. This short note summarises its main findings.