There has been significant progress in recent years in improving the energy efficiency of new buildings, driven by technological improvements and various regulatory requirements and policy initiatives, and there is beginning to be a critical mass of very low energy buildings or even zero energy buildings in various regions of the world. However, a large percentage of existing buildings are significantly less efficient than most newly constructed buildings, and have major opportunities for improvement. Energy consumption from buildings that had been constructed prior to the implementation of recent improved energy performance regulations will continue to consume significant amounts of energy long into the future. Determining the most effective set of policies to significantly improve the existing building stock is a key challenge for energy policy makers around the world.