Commissioning is the process of ensuring that systems are designed, installed, functionally tested, and capable of being operated and maintained as the engineers' design intended. 

Retro-Commissioning is the process of commissioning an existing building that has never been commissioned before.  There are many reasons to retro-commission a building including high energy bills, poor air quality, occupant discomfort and equipment malfunctions.  The process is the same as commissioning a newly constructed building and includes developing a commissioning plan, convening Cx team meetings, developing the pre-functional checklists and functional performance testing documentation, overseeing TAB and system start-up, compiling systems manuals and writing the final commissioning plan.  Building owners also have the option of a back check of design documents prior to the construction phase and a site visit 10 months post construction.