Building heating ventilation and air-conditioning (HVAC) systems are designed to operate at the peak load, which only occurs in a very short period of time throughout the year. One of the most effective ways to improve building energy efficiency is to utilize the variable frequency drives (VFDs). They are widely used in the HVAC field, including fans, pumps, compressors, etc. In a VFD-equipped system, the VFD adjusts the speed of one or more motors based on the system load requirements and operation schedule, resulting in a dramatic cut in energy consumption. This article systematically introduces the application of VFDs in HVAC systems, including heating systems, ventilation systems, cooling systems, and refrigeration systems in commercial and industrial buildings. First, the principles are introduced for VFDs, variable speed fans, pumps, and compressors. Next, the control application of VFDs on each type of system (equipment) is summarized. In this section, the schematic diagram of each system as well as the detailed control strategy for each application is presented. In addition, the application of VFD on fault detection and diagnosis (FDD) is introduced. Lastly, some technical issues and concerns are discussed with potential solutions provided.