Monitoring of respiratory function in the intensive care unit uses different techniques in spontaneously breathing patients and during mechanical ventilation. The simplest and most efficient means are clinical observation, non-invasive monitoring of respiratory movements and frequent blood gas analysis. The respiratory force and reserves can be assessed by measuring tidal volume, vital capacity and maximal inspiratory force (in cm H2O or mm Hg), and these values are helpful in deciding on ventilatory assistance or extubation. During mechanical ventilation the function of the respirator must be monitored by appropriate sensors and alarms. In addition, the measurement of functional residual capacity, compliance and distribution of ventilation, as well as changes therein with the application of positive end-expiratory pressure (PEEP), can contribute to evaluate of appropriate therapy and prognosis of pulmonary failure.