In the intensive care setting, monitored data relevant to the output, efficiency, and reserve of the respiratory system alert the clinician to sudden untoward events, aid in diagnosis, help guide management decisions, aid in determining prognosis, and enable the assessment of therapeutic response. This review addresses those aspects of monitoring we find of most value in the care of patients receiving ventilatory support. We concentrate on those modalities and variables that are routinely available or easily calculated from data readily collected at the bedside.