This paper describes the concept of using simulation as a tool for performance validation and energy analysis of HVAC systems. Recent advances in control system technology, including the development of open protocols such as BACnet(TM) have made sensor and control signal information from various components and subsystems in a building more accessible. This development has created significant potential for improving the monitoring and supervision of building systems in order to optimize operational performance. The paper describes one way of making use of this new technology by applying simulations, configured to represent optimum operation, to monitored data. The idea is to use simulation predictions as performance targets with which to compare monitored system outputs for performance validation and energy analysis. The paper presents results from applying the concepts to a large dual-duel air-handling unit installed in an office building in San Francisco. (C) 2000 Elsevier Science S.A. All rights reserved.