Energy expenditures represent approximately 10% of the global gross domestic product (GDP), second only to healthcare costs. Globally, the residential and commercial building sector accounts for 21% of total energy consumption. In the United States, the building sector accounts for a much larger share: 40% of the country's total energy consumption. With an increased global focus on energy conservation, the building sector presents a ripe target for introducing energy efficiency. While advancements have been made in building architecture and materials, the length of time needed for mass adoption of these technologies renders them ineffective in the short and medium term. Retrofitting existing buildings is a more desirable option, resulting in immediate energy savings while preserving the building structure. In this paper, we discuss several existing and ongoing efforts directed toward promoting energy efficiency in buildings, focusing specifically on building retrofits, and analyze their energy savings potential. This collection of building technologies spans several categories and represents various building energy end-uses. We also discuss the challenges involved in developing such energy-efficient building technologies.