This chapter explains a relatively new process for gathering and analyzes energy data, “Retro-commissioning (RCx). The techniques utilized by the experienced RCx agent can serve equally well for those searching for the source of room comfort problems, or those involved in a forensics effort to discover why a system isn’t operating properly. The Planning Phase report should be basic and to the point. It should contain a list of potential projects selected from a checklist of normally discovered projects for the specific type facility surveyed. Many times a RCx agent suspects the existence of a problem such as excessive introduction of outside air or too much chilled water flow through an air handler coil. The relatively low implementation costs borne by building owners to install the low-cost improvements discovered by retro-commissioning professionals have resulted in a typical simple payback period of less than three years.