Intelligent management of pulmonary disease requires a fundamental understanding of the complex interrelationships between multiple elements that interact to maintain homeostasis in the respiratory system. This chapter discusses the physiologic basis for pulmonary function testing and the use of these tests to diagnose disease, quantitate functional impairments, and follow the effects of treatment. Figures illustrate the relationships between lung volume, airway conductance, and airway resistance; the relationship between forced expiratory volume and time (spirogram); the relationship between spirometry and maximum expiratory flow volume; dynamic airway compression; patterns of abnormalities seen on flow-volume curves; lung volumes and capacities; pressure-volume relationships in health and disease; and the clinical assessment of lung function. Tables list the capacities and volumes of gas contained in the lungs during various breathing maneuvers, conditions associated with alterations in diffusing capacity, and recommendations for tests for various clinical scenarios. This review contains 7 highly rendered figures, 3 tables, and 86 references.