In this chapter, energy e—ciency measures are combined with renewable energy technologies to retro‘t existing homes to be low-energy or zero-energy buildings. In particular, optimization methodologies are introduced to determine the best combination of energy e—ciency strategies and solar thermal and photovoltaic (PV) systems to reduce the reliance of residential buildings on primary energy resources. Both source energy use and life cycle cost are accounted for in the optimization procedure to retro‘t existing homes to zero-energy buildings. In addition to the optimization techniques, general guidelines are provided to select the most cost-ešective energy e—ciency measures suitable for existing residential buildings. In particular, a set of measures as well as PV system sizes are provided for typical homes located in representative U.S. climates.