Retro-commissioning is the process of commissioning an existing facility due to system upgrades or a desire for increased efficiency. Our original market niche was troubleshooting and optimizing existing buildings and this is where our hands-on experience with dynamically operating equipment began. During most retro-commissioning processes, there are no designers or contractors and as such, the retro-commissioning agent must be experienced in all aspects of facility operation. Due to the fact that many of our core personnel came from the performance contracting industry where the focus was on improving building functioning, Facility Dynamics Engineering is highly skilled in the retro-commissioning process. Additionally, it was through our early experiences with existing building optimization that the seed for the creation of our constant commissioning software originated. The several man-years spent developing our software exemplifies our diligence and efforts to meet the challenges of creating efficient and fully-functioning buildings. As a result, FDE is highly adept at optimizing existing facilities and we are the firm of choice for retro-commissioning projects.