A Building Management System (BMS) is a computer-based control system installed in buildings that controls and monitors the building's mechanical and electrical equipment such ventilation, lighting, power systems, fire systems, and security systems. A BMS consists of software and hardware; the software program, usually configured in a hierarchical manner, can be proprietary using such protocols as C-bus, Profibus, etc. Recently however new vendors are producing BMSs that integrate using Internet protocols and open standards like DeviceNet, SOAP.XML, BACnet, Lon and Modbus.

A BMS is more typical in a large building. Its core function is to manage the environment temperature, carbon dioxide level and humidity within a building. As a core function in most BMS systems, it controls the production of heating and cooling, manages the systems that distribute this air throughout the building, and then locally controls the mixture of heating and cooling to achieve the desired room temperature. A key secondary function is to monitor the level of human generated C02, mixing in outside air with waste air to increase the amountof02 while also minimizing heat/cooling losses.

Systems linked to a BMS typically represent 40% of a building's energy usage; if lighting is included this number approaches 70%. BMS systems are a critical component to managing energy demand. Improperly configured BMS systems are believed to account for 20% of building energy usage, or approximately 8% of total energy usage in the United States.