The original idea of Fitch, who conceived of the building wall as a modulating membrane, has been elaborated in this article into a model of the Man Environment System. Four subsystems, reap. the physiological, the psycho logical, the social and the physical, are identified and described in terms of feedback-mechanisms, that complement each other in particular ways. Some of these relations are described. The relevance of the model for Architectural-Psychological research is discussed in terms of integrative power as well of predictive power.