Energy performance indices are used around the world to evaluate and monitor residential and commercial building energy performance during design, construction, renovation, and operation. Building energy performance is sensitive to the engineered system, O&M practices, and occupant needs, each of which are largely independent factors requiring more than one index to describe and manage the building’s energy performance. This presentation analyzes the merits and weaknesses of common indices and introduces the O&M Index, which is the ratio of the energy consumption at the meter to the simulated energy performance, calibrated for the actual operating conditions of the building.