Building performance simulation in the context of energy efficient buildings and districts is still mostly used for code compliance checking. However, when used appropriately building performance simulation has the potential to help reduce the environmental impact of the built environment, to improve the indoor environmental quality and productivity, as well as facilitating future innovation and technological progress in construction. For that to become widely available, the building simulation community still has to overcome several challenges both in terms of technology and application methodology. Several of these challenges relate to the need to provide better design support. Issues include early phase design support, multiscale 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-physics (particularly inclusion of electrical power flow modeling), and integration in the construction process (using building information modeling (BIM), process modeling, etc). Another group of challenges (and opportunities) relate to the need to provide support for building operation and management. Here the issues include accurate in-use energy consumption prediction and model predictive control.