In this chapter, energy efficiency measures (EEMs) are combined with renewable energy technologies to design and retrofit buildings or communities to reach low-energy, zero-energy, or carbon-neutral ratings. In particular, optimization methodologies are introduced to determine the best combination of energy efficiency strategies and renewable energy systems to reduce the reliance on primary energy resources. Both source energy use and life cycle cost are accounted for in the optimization procedure to design zero-energy buildings or carbon-neutral communities. In addition to the optimization techniques, specific case studies are described to design and retrofit buildings and urban communities to achieve low energy consumptions and low carbon emissions.