In this paper we approach the subject of ventilation and occupant behavior in multifamily buildings by asking three questions: 1) why and how do occupants interact with ventilation in an apartment building, 2) how does the physical environment (i.e., building characteristics and climate) affect the ventilation in an apartment, and 3) what methods can be used to answer the first two questions. To investigate these and other questions, two apartment buildings in Chicago were monitored during the 1985 - 1986 heating season. In addition to collecting data on energy consumption, outdoor temperature, wind speed, and indoor apartment temperatures, we conducted diagnostic measurements and occupant surveys in both buildings. The diagnostic tests measured leakage areas of the individual apartments, both through the exterior envelope and to other apartments. The measured leakage areas are used in conjunction with a multizone air flow model to simulate infiltration and internal air flows under different weather conditions. The occupants were questioned about their attitudes and behavior regarding the comfort, air quality, ventilation, and energy use of their apartments. This paper describes each of the research methods utilized, the results of these efforts, and conclusions that can be drawn about. ventilation-occupant interactions in these apartment buildings. The major conclusion of this work is that a multidisciplinary approach is required to understand or predict occupant-ventilation interactions. Such an approach must tske into account the physical characteris tics of the building and the climate, as well as the preferences and available options of the occupants.