Abstract. Measuring the energy efficiency of buildings and its confrontation with the current Building Energy Simulations now faces knowledge of what is commonly called "occupancy". This work has been made in order to implement a monitoring system on a research demonstrator building at DLRCA in Angers (France). The goals were first to know the occupancy as input data of models but also to build occupancy models. Occupancy can be defined as all the action of occupants that affect building energy efficiency. The chosen monitoring deals with its presence, lightning, windows opening and internal gains. It seems that the use of an Infra-red detector allows a accuracy of 5 min in the detection of presence. The use of dry contact sensors allows the detection of five different rates of slide windows opening that can affect temperature decrease. Light sensors seem to be efficient to detect artificial lighting states when correctly configured.