In recent decades, architects have turned to computer simulation with the hope of designing more functional, sustainable, and compelling buildings. In such efforts, it is important to regard buildings not merely as static structures, but rather as complex dynamic systems driven by highly stochastic elements including the weather and human behavior. In this chapter, we describe how simulation has impacted architectural design research and practice. A multitude of simulation tools have been developed to model specific aspects of a building such as thermodynamics, daylight, plug loads, crowd behavior, and structural integrity under internal and external loads. Yet numerous challenges remain. For example, although many factors influencing buildings are interdependent, they are often analyzed in isolation due to the development cost associated with integrating solvers. A systems approach combining visual programming with state-of-the-art modeling and simulation techniques may help architects and building scientists combine their expertise to produce integrated complex systems models supporting emerging paradigms such as generative design.