Therapeutic aerosols are commonly used in mechanically ventilated patients, mainly to deliver bronchodilator drugs. Because ventilator-supported patients often receive several different classes of medications parenterally, the inhaled route of administration is particularly useful as an alternate route for drug delivery. Both nebulizers and metered-dose inhalers (MDIs) can be adapted for use in ventilator circuits. Traditionally, nebulizers have been used to deliver bronchodilators, antibiotics, and surfactant in mechanically ventilated patients, whereas MDIs chiefly have been used to deliver beta-adrenergic and anticholinergic bronchodilators (1). In the past, the combination of an MDI with an elbow adapter was considered to be ineffective (2) due to aerosol impaction in the ventilator circuit and endotracheal tube (3). However, recent studies (4–6) have clearly established the efficacy of MDIs for delivering bronchodilators in mechanically ventilated patients, with the caveat that the technique needs to be carefully executed. With a better understanding of the many factors influencing aerosol delivery and deposition in mechanically ventilated patients, it is possible to provide a scientific basis for inhaled bronchodilator therapy in these patients.