In the new era of climate change, cities play an important role in reducing energy use and mitigating climatic disastrous effects. The current research and planning practice on the “good urban form” in terms of city energy performance are still at the very early stage. The City Energy Lab (CEL) aims to find better solutions for energy efficient and resilient urban development in global contexts by exploring three aspects of city energy: science, design and policy. The lab examines the relations between urban form and energy in global cities with urban energy modeling tools, develops Multi-Disciplinary Design Optimization (MDO) based AI Design (Artificial Intelligence Design) methods to integrate energy performance evaluation into planning and design processes, and provides urban policy suggestions for low carbon urban development. The lab places these research agendas on American, Asian and European cities in different social, historical, cultural and economic contexts. With the approach of city as living scientific laboratory, we are testing the ideas of “science for design” and “design in science”, and further proposing “policy with science and design” to address city energy challenges.