This article discusses relationships between building performance simulations and design decisions and how building performance predictions can improve the design outcomes. The first part of the article discusses why we need to quantify building performance and predict how a building as a whole or its components will function. Then, relationships between Building Information Modeling (BIM) and analysis software applications are discussed, where best practices for developing BIM models that are suitable for different types of simulations are discussed. Lastly, two case studies are used to illustrate this process. The first study reviews curtain wall energy performance for a healthcare facility located in a mixed humid climate and daylighting analysis. The second case study discusses comprehensive analysis for an academic research building focusing on site and orientation studies, solar exposure, investigation of performance of shading devices and daylighting analysis.