In an attempt to enhance the performance of the built environment, models are being used everywhere to better design or operate buildings. Whole-building energy models are being used for design trades and for compliance (e.g. LEED) in design firms and these or reduced order versions are used for online predictive optimization or fault diagnostics after the building is commissioned. The models used vary greatly in their complexity, size (the number of physical attributes of the building they intend to capture), computational cost and flexibility (access to dynamic states etc.). In this talk we discuss different modeling approaches, uncertainty in these models and how to overcome some of the computational challenges that different models introduce. We will also discuss the notion of closing some gaps between different modeling methods to provide seamless methods to use the same models across the design and operation stages of a buildings life cycle.