The ventilatory control system, although not yet fully understood, is a complex interaction of volitional and reflex mechanisms extending from the cerebral cortex to receptors in the lungs and chest wall. Measurements of ventilatory control mechanisms can be made at rest, during exercise, during sleep, and during hypercapnic or hypoxic stimuli. Measurements of ventilation, ventilatory pattern, and mouth occlusion pressure at rest provide valuable information on ventilatory control mechanisms; these measurements can be extended by measurements of diaphragmatic EMG (indicating central neural output) and measurements of ventilation and mouth occlusion pressure during CO2 or hypoxic stimulation. Clinical assessment of ventilatory control abnormalities requires an understanding of the basic features of the control system and a logical application of the available tests.