During spontaneous breathing, respiratory muscle pressure (Pmus) waveform is determined by a complex system consisting of a motor arm, a control centre and various feedback mechanisms that convey information to the control centre. In mechanically ventilated patients, the pressure delivered by the ventilator (Paw) is incorporated into the system that controls breathing and may alter Pmus, which in turn modifies the Paw waveform. Thus, the response of the patient's respiratory effort to Paw and the response of Paw to patient effort constitute the two components of the control of breathing during mechanical ventilation. The response of Paw to patient effort depends on: 1) the mode of ventilatory support; 2) the mechanics of the respiratory system; and 3) the characteristics of the patient's respiratory effort. On the other hand, the response of patient effort to Paw is mediated through four feedback systems: 1) mechanical; 2) chemical; 3) reflex; and 4) behavioural. It follows that in mechanically ventilated patients the ventilatory output is determined by the interaction between the function of the ventilator and the patient's breathing control system. This interaction should be taken into account in the management of mechanically ventilated patients.