Inhaled therapy is routinely employed during mechanical ventilation. Several factors affect aerosol delivery: the aerosol device, particle size, ventilator parameters, ventilator circuit and hygrometry. Non invasive ventilation is commonly used for treatment of exacerbations of chronic obstructive pulmonary disease. However, there are few data concerning the factors affecting aerosol delivery during this mode of ventilation. Optimal aerosol delivery during mechanical ventilation depends on the aerosol device, the respirator circuit and settings, and the patient himself.