Deep energy retrofits, which can save upwards of 50 percent or more of a building’s energy consumption, hold the key to enabling significant building energy use reductions and operational cost savings. They could also bring federal agencies into compliance with federal energy efficiency mandates. While this opportunity has long been recognized by energy service companies (ESCOs) and the General Services Administration (GSA), deep energy retrofits are still uncommon. There are several challenges, big and small, that have been explored over the past four years as part of the GSA’s National Deep Energy Retrofit (NDER) program. And as proof of concept, the GSA has anted up with a group of buildings and has demonstrated the power deep energy retrofits hold. However, there is still more work to be done.