This chapter provides a concise overview of mechanical systems used for heating, ventilation and air conditioning (HVAC) in residential and commercial applications with emphasis on energy efficient equipment. Components and systems considered include vapor compression systems, chill water systems, heat rejection systems, radiant heating systems, gasfired heating systems, ventilation systems, design processes and strategies, equipment sizing, review of codes and standards, system integration, and future trends of these technologies. The focus of the chapter is on primary mechanical HVAC systems.