The dissertation addresses the limitation of current building energy simulation programs in accounting for occupant behaviors, which have been identified as having significant impact on the overall building energy performance. It introduces a new simulation methodology using an agent- based modeling approach that helps to both predict real-world occupant behaviors observed in an operating building and to calculate behavior impact on energy use and occupant comfort. A series of experiments has been conducted using the new methodology and yielded simulation results that not only distinguish themselves from current simulation practices, but also uncover emerging phenomena that enhance the insights on building dynamics.