The DOE Weatherization Assistance Program enables low-income families to permanently reduce their energy bills by making their households more energy efficient. The DOE program guidelines specify that a variety of energy efficiency measures are eligible for support under the program. The measures include insulation, space-heating equipment, energy-efficient windows, water heaters, and efficient air conditioners. This book explores the Department of Energy’s (DOE’s) Weatherization Assistance Program, with a focus on its background, brief history of funding, program evolution, program activity, and program assessments and benefit-cost evaluations.