The interaction between the patient and the ventilator is complex,especially in a "semi-open" system as for noninvasive ventilation(NIV). Air leaks around the mask are likely to occur, and they affect patient-ventilator synchrony. Several variables may be responsible for the mismatch between the start of the neural output and that of ventilatory aid during NIV. The most common mode of ventilation is pressure support ventilation (PSV), which may result in a number of inspiratory efforts not being followed by ventilator aid. New modes of ventilation, such as proportional assist ventilation, maybe useful in improving patient tolerance to ventilation without affecting clinical outcome. The ventilatory settings are important during PSV to determine the synchrony. The inspiratory trigger function may be influenced by the amount of leaks, whereas a better synchrony may be achieved if the termination of the inspiratory phase is time cycled instead of flow cycled. A high pressurization rate results in poor compliance. Care should be paid in the choice of the interfaces because leaks in the system are associated with a substantial breath-to-breath inspiratory variation independent from the patient effort. Last, NIV should be delivered with turbine- or piston-based ventilators that are able to compensate for air leaks. With respect to the problem of sedation, we point out the importance of optimizing the environmental conditions, avoiding excessive light and noise, assuring patient comfort, and providing reassurance. When sedation is needed, we suggest the use of low doses of analgesics and neuroleptic agents in selected cases.