This paper establishes the need for testing and development of automotive heater/defroster systems in the real world and outlines the parameters which must be considered in developing and conducting repeatable and meaningful test procedures. A specific test objective is defined and the parameters identified are individually discussed with respect to that objective. Alternative approaches are then evaluated and a specific test procedure is developed. Results from the tests conducted are presented and analyzed to illustrate the scope and relevance of the information generated. It is concluded that the developed procedure is not only meaningful in terms of evaluating individual vehicle/system performance, both on an absolute and relative basis, but also in facilitating correlation between human comfort characteristics and objective test data. Such information can ultimately be used to establish comfort criteria and design parameters as well as performance target specifications.