In this module, an overview of the field of building performance simulation applied to the operational phase of the building lifecycle is provided. Firstly, the different approaches to constructing building energy models is described (white box, grey box, black box) with their pros and cons. The role of the International Building Performance Simulation Association is introduced with an evolution of the evolving focus of building simulation through the different conference topics. Different options and experiences for the use of simulation addressing operation and HVAC systems are reviewed with an emphasis on the simulation tools used and the integration in current BMS systems and smart grid. Finally, we introduce the future of building performance models through co-simulation and model exchange using the Modelica language and the Functional Mockup Interface.