The task is for COMIS to develop a reliable and well running multizone infiltration model on a modular base. This model does not only take crack flow into account, but also covers flow through large openings, single sided ventilation, cross ventilation and HVAC-systems. The model contains a large number of modules, which are peripheral to a steering program. COMIS can be used as a basis for future expansions in order to increase the capabilities of simulating buildings or their type of construction, in the field of ventilation, heat flow and spread of pollutants. To develop the different modules, small task groups were formed to work on particular problems. Each COMIS team member works on several task groups and might be responsible for one or more of them. Almost three years ago, the first announcement was send to colleagues to inform them about the joint research project planned to develop a multizone infiltration model at LBL. Even though this kind of co-operation is well known in other fields of research, e.g., high energy physics; for the field of building physics, it seems to be a new approach to be engaged in a research project, one individual or country otherwise would not be able taken by the research society. Due to the internationality of the group, several national and international research programmes are co-ordinated with the COMIS workshop. This paper describes the COMIS workshop as well as the outline of the COMIS model.