This paper aims to provide a general view of the background and current state of building performance simulation, which has the potential to deliver, directly or indirectly, substantial benefits to building stakeholders and to the environment. However the building simulation community faces many challenges for the future. Several challenges relate to the need to provide better design support. Issues include early phase design support, multi-scale approaches (from construction detail to district level), uncertainty and sensitivity analysis, robustness analysis (employing use and environmental change scenarios), optimization under uncertainty, inverse approach (to address "how to" instead of being able to answer "what if" questions), multi-ph...