This paper catalogs deep retrofit activities in existing buildings. Our review of completed ultra-low energy building (ULEB) retrofit projects in the residential and commercial sectors shows that deep energy savings are technically feasible. While some buildings need extensive interventions requiring significant investments of time and money, others can meet aggressive targets at relatively low cost. Projects can use either leading-edge technologies and techniques or tried-and-true weatherization methods. Some building owners and occupants are willing to adopt new behaviors and practices to meet energy efficiency goals. We identify opportunities to increase the number of existing ULEBs through market development, advanced technologies, and new retrofit techniques. Moving forward, further work is needed to develop the best models for delivering savings and standardized approaches that will work at scale.