Using extensive field survey data acquired over the past seven years at the Solar Energy and Building Physics Laboratory at EPFL in Switzerland, models of occupants’ presence, opening and closing of windows and the raising and lowering of blinds have been developed. These new models, together with existing models of lighting and appliance use, have been integrated within a new urban energy modelling tool, called CitySim. In this paper 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 simulations of the impact that occupants’ behaviour may have on the indoor environment in buildings’ as well as on buildings’ energy demands at the urban scale. We close this paper by discussing a more general framework for the more comprehensive representation of occupants and their activities in building and urban simulation tools.