Energy-related occupant behavior is crucial to the design and operation of low-energy buildings. This chapter introduces state-of-the-art methods, tools, and datasets for quantifying occupant impacts on building energy use and occupant comfort. The chapter begins with an overview of how occupants can influence building environments and energy performance, and highlights gaps in the abilities of building energy simulation programs to represent these influences. Next, state-of-the-art methods and modeling tools that enable more sophisticated occupant behavior simulation are reviewed, along with the most prominent datasets available to support quantitative behavior model development. Then, an overview of application areas for occupant behavior modeling tools and datasets across the building life cycle is presented, and example applications are demonstrated through three case studies. The chapter concludes by identifying emerging opportunities and challenges surrounding the use of occupant behavior simulation to support the design and operation of low-energy buildings that foster greater occupant satisfaction.