HVAC systems are comprised of the heating, ventilation, and air-conditioning systems in domestic and commercial buildings. These mechanical systems provide thermal comfort and air quality in indoor spaces. Types of HVAC systems include air conditioners, heat pumps, furnaces, boilers, rooftop units, chillers, and packaged systems. The HVAC system plays an important role in the energy efficiency of buildings. HVAC systems account for 39% of the energy used in commercial buildings in the United States. Consequently, almost any business or government agency can realize significant savings by improving its control of HVAC operations and improving the efficiency of the HVAC system. The uses of high-performance HVAC equipment can result in considerable energy, emissions, and cost savings (10%–40%). Whole building design coupled with an “extended comfort zone” can produce greater savings (40%–70%). Extended comfort includes employing concepts, such as providing warmer, drier air using desiccant dehumidification in summer, or cooler air with warmer windows and walls in winter. In addition, high-performance HVAC can provide increased thermal comfort, and contribute to improved indoor environmental quality (IEQ). This chapter describes different types of air conditioning systems that are the important component of efficient HVAC systems. The chapter also focuses on the technology overview, energy consumption, energy saving strategies, and the role of HVAC system in energy-efficient buildings.