This paper summarizes the physical basis for ROOMFIRE, a new single-compartment single-vent generic fire model. ROOMFIRE solves the conservation equations of mass and energy for the lower and upper gas layers in the compartment. Various auxiliary equations are solved simultaneously. Thus, ROOMFIRE calculates primary and secondary state variables as a function of time. The former define the geometry, temperature, and composition of the two gas layers. The latter include wall temperatures, mass flows through the vent, and entrainment into the fire. One or multiple fires are specified in separate modules that are external to ROOMFIRE. Therefore, ROOMFIRE can be used for different types of fires, by using it in combination with the appropriate fire modules.