The decision whether to wean is a challenge to physicians, because in as many as 10|X% to 20|X% of cases, weaning fails to avoid reintubation or unnecessarily prolongs ventilation. Noninvasive mechanical ventilation (NIMV) is a readily available method for supplying ventilatory support. Although the advantages for NIMV are substantial in some scenarios (long-term home mechanical ventilation, sleep apnea syndromes, acute respiratory failure in patients with chronic obstructive pulmonary disease), the evidence for its use during weaning is lacking. NIMV can be used in weaning in two different ways: first, as a supportive ventilatory method, which allows for early withdrawal of the endotracheal tube, and, second, as a ventilatory aid in patients in whom extubation fails. Whether the different ways of providing NIMV account for clinical differences in outcome remains uncertain.