Building commissioning is a method of risk reduction for new construction and major renovation projects to ensure that building systems meet their design intent, operate and interact optimally and provide the owner what he or she expects. This systematic process typically includes building HVAC, controls, lighting, hot water, security, fire, life and safety systems.

Total building commissioning often includes additional essential buildings systems such as the building's exterior wall, plumbing, acoustical and roofing systems.  Commissioning these additional systems can reduce moisture penetration, infiltration and noise problems, and contribute to the building's energy and resource efficiency and occupant productivity.

Successful building commissioning results in optimal energy efficiency, indoor environmental quality, reduced change orders during construction, extended systems life and reduced operation and maintenance costs, often paying for itself before construction is completed. To be most effective, building commissioning begins in the planning phase and continues through design, construction, startup, acceptance, training and the warranty period, and continues throughout a building's life cycle.

The commissioning process can be applied to existing buildings that have never been commissioned to restore them to optimal performance. Retro-commissioning is a systematic, documented process that identifies low-cost operational and maintenance improvements in existing buildings and brings the buildings up to the design intentions of its current usage.

Retro-commissioning typically focuses on energy-using equipment such as mechanical equipment, lighting and related controls and usually optimizes existing system performance, rather than relying on major equipment replacement, typically resulting in improved indoor air quality, comfort, controls, energy and resource efficiency.

Retro-commissioning typically includes an audit of the entire building including a study of past utility bills, interviews with facility personnel. Then diagnostic monitoring and functional tests of building systems are executed and analyzed.  Building systems are re-tested and re-monitored to fine-tune improvements.  This process helps find and repair operational problems.  The identification of more complex problems are presented to the client as well.  A final report, retro-commissioning plan and schedule are then given to the owner.

Building systems can be purchased from different vendors, installed by different contractors and operated by different facilities staff, who are under pressure to resolve occupant complaints about comfort.  Quick fixes may resolve an individual complaint, but can lead to other systems becoming out of balance and losing the persistence of benefits from initial building commissioning or retro-commissioning. Additionally, building systems require periodic analysis and adjustment.

Ongoing commissioning, also referred to as "retro-commissioning follow-up," is continual retro-commissioning focusing on the persistence of completed improvements. Ongoing commissioning involves regularly scheduled sessions with the building occupants along with operation and maintenance personnel. This process incorporates monitoring and analysis of building performance data provided by permanently installed metering equipment to verify building performance, the satisfaction of the facilities management and staff, and the extent of actual savings.