It is essential to monitor respiratory functions in unstable patients. Although there is no good substitute for careful clinical observation of a patient's respiratory status, the technology available today is quite helpful in evaluating various respiratory parameters. In the ICU, monitoring the respiratory status may help determine the need for mechanical ventilation. It may also monitor ventilator safety, determine disease progression, and help decide when to wean the patient from mechanical ventilation. Taken together, various respiratory parameters provide useful information about trends in oxygenation, ventilation, and mechanics. The article reviews current concepts of respiratory monitoring of critically sick patients.