In health, breathing is regular and the respiratory rate is sufficiency constant to be useful as a vital sign of health and disease. This regularity depends on a complex interplay of chemical and neural control systems that operate automatically to reset the rate and depth of breathing as changes occur in posture and activity, to adjust the level of ventilation so that changes in gas tensions and pH in the blood and in the brain intersitial fluid are exceedingly modest despite wide swings in metabolic rate and in environmental conditions, and to coordinate ventilation and circulation so that the requirements of individual tissues for O2 delivery and CO2 removal are satisfied. Two broad categories of disorders can result from malfunction of these systems (Table 1): (1) disproportionate ventilation (too high or too low) for the level of metabolic activity, thereby producing severe abnormalities in blood gas tensions or in acid-base balance, and (2) an irregular breathing pattern without eliciting gross changes in blood gas tensions or in acid-base balance. Because of the complexity of the control system, each of these categories represents a final common pathway that can be produced in different ways. In this presentation, we will attempt to describe the general features that characterize the operation of the control system and some new technics that make it possible to trouble-shoot the malfunctioning system in order to identify the mechanism(s) responsible for the abnormality in breathing pattern.