the simplest indicator of the adequacy of ventilatory control. Where there is little or no mechanical abnormality, an elevation of the CO2 tension is an indication of inadequate ventilation and impaired control mechanisms. The respiratory muscles play a crucial role in the ventilation process. In Chapter 4 tests to evaluate the strength and endurance of the respiratory muscles are described. The main function of the lung is the exchange of O2 and CO2 between the ambient air and the capillary blood. Diffusion characteristics of the alveolo-capillary membrane and ventilation-perfusion mismatch play an important role in gas exchange. These items are discussed extensively in Chapters 6 and 7. Finally, exercise testing, where all the aforementioned systems are subjected to stress, is reviewed in Chapter 8. Unfortunately, a chapter dealing with reversibility and provocation tests in patients with asthma to study hyperreactivity of the airways could not be included in this Monograph. However, we are convinced that the most important issues concerning lung function testing are reviewed. H. STAM, R. GOSSELINK ix