Ongoing urbanization and ever-growing harmful environmental impacts from urban areas necessitate a sustainability transformation in cities. However, cities are also centers of wealth creation and consumption, which both drive environmental degradation. It is clear that cities need to re-establish themselves as low-energy/low-carbon systems, but the transformation is complex in many ways and time is running out. This Special Issue, "Energy Efficient Cities of Today and Tomorrow", seeks to provide a more profound understanding of the future energy requirements of urban areas and low-energy and low-carbon cities. The published papers range from macro-level assessments of cities manifesting themselves as forerunners in their environmental work to micro-level studies of pro-environmental attitudes and their impacts on individual emissions, a carbon footprint impacts of sharing of goods and services.