The 26th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2013) will be held at Guilin, China, July 16-19, 2013. The Organizing Committee of ECOS 2013 invites all scientists, researchers and engineers in the fields of energy systems to participate in this important event to contribute and share their ideas and research results with other colleagues. The conference features 18 tracks on various aspects of energy systems. Abstracts on these tracks are welcome.