The measurement of the thermal performance of a heat exchanger presents a number of difficulties. Test methods, timescales and attainable accuracies have long been the subject of animated debate. A number of pressures, economic and practical, are working in combination to make accurate determination of component thermal performance, operating singly or in groups, more important. A review of the fundamental factors involved in calorimetry is presented together with a review of likely accuracy of measurement. Instrumentation problems and the uncertainty of physical data are discussed. Instrumentation and control techniques appropriate to reliable calorimetry are briefly reviewed. The development of a modular approach to calorimeter construction is described. The solution outlined provides complete flexibility and fulfils most present needs for testing and acquiring data on the thermal performance of all automotive heat exchangers.