Using extensive field survey data acquired over the past 8 years at the Solar Energy and Building Physics Laboratory at EPFL in Switzerland, comprehensive models of occupants’ presence, opening and closing of windows and the raising and lowering of blinds have been developed. These new models have been integrated within a new urban energy modelling tool, called CitySim. In this article, we describe briefly the structure of CitySim together with the means for representing occupants’ presence and behaviour, both deterministic and stochastic. For a hypothetical scenario, we then go on to present results from simulations of the impact that occupants’ behaviour may have on the indoor environment in buildings as well as on buildings’ energy demands. From this we conclude that occupants’ behaviour has a significant impact (of the order of a factor of two) on buildings’ energy demands and that individuals’ diversity has a yet greater impact.