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.