A large proportion of the world’s energy consumption is spent in an effort to maintain a comfortable and healthy indoor environment. As a consequence reductions in the energy consumed to climatise buildings are instrumental to the efforts of reducing energy related CO2 emissions and alleviating the European energy import dependencies. Whole building simulations of indoor environment and energy consumption are becoming more and more used in the design phase of buildings. Previously the simulation of physical factors such as transmission and ventilation heat losses has received a lot of attention. As a consequence, most programs are capable of accurate simulations of the physical properties of a building. However, even though the occupants’ control of the various systems in the building has a significant impact on the energy consumption and the indoor environment, only few studies have focused on the behaviour of their occupants. As a consequence, there is a need to investigate occupants’ interactions with building controls, such as opening of windows, adjustments of heating set-points, use of solar shading, etc. Some models of occupants’ interactions with operable windows do exist, but these are based on measurements in offices, and they only take thermal comfort into account. The work described in this thesis mainly focused on the window opening and heating behaviour of occupants in Danish dwellings. Also the use of solar shading and artificial lighting has received some attention. The control related behaviour of occupants was found to have a substantial impact on the energy performance of a building. This becomes increasingly important in buildings designed using the adaptive model of thermal comfort, where occupants are encouraged to interact with building controls. It was found that determination of acceptable thermal conditions with the adaptive model may result in significant energy savings and at the same time will not have large consequences for the mental performance of the occupants. Large differences in the behaviour patterns of occupants were found between dwellings. The time of day had a great effect on the behaviour patterns in the investigated dwellings. This effect was significant at similar environmental conditions, suggesting that environmental variables alone can not explain all the variance in the observed behaviour. The results showed that the behaviour of the occupants was driven by a variety of variables, including thermal comfort, perception of air quality and other IEQ variables, weather and physical aspects of the dwelling. Based on observation of real behaviour, a definition of occupant behaviour patterns in building simulation programs was proposed. The proposed model was implemented into the simulation environment IDA ICE and compared to a reference simulation, which emulated a simulation as it could have been performed by a designer. There were large differences in the simulated indoor environment between the two simulations, which resulted in considerably lower energy consumption in the reference simulation. Since the definition is based on observation of real behaviour, it will significantly increase the validity of the simulation result and ensure that the results are closer to reality, when implemented into simulation programs. Furthermore, it will enable designers to better assess the effects of the occupant’s behaviour and thereby the effects of different designs.