Energy-related occupant behavior is crucial to design and operation of energy and control systems in buildings. Occupant behaviors are often oversimplified as static schedules or settings in building performance simulation ignoring their stochastic nature. The continuous and dynamic interaction between occupants and building systems motivates their simultaneous simulation in an efficient manner. In the past, simultaneous simulation has relied on cosimulation approaches or customized source code changes to building simulation programs. This paper presents Buildings.Occupants, an open-source package implemented in Modelica, for the simulation of occupant behaviors of lighting, windows, blinds, heating and air conditioning systems in office and residential buildings. Examples were presented to illustrate how the models in the Occupants package are capable to simulate stochastic occupant behaviors. The major contribution of this work is to introduce the equation-based modelling approach to simulate occupant behaviors in buildings, and to develop an open-source Occupants package in the Modelica language.