An increasing number of pharmacologic agents, including bronchodilators, prostaglandin, proteins, surfactant, mucolytics, and antibiotics are administered to mechanically ventilated patients by the inhalation route. To achieve a therapeutic effect, adequate amounts of an inhaled agent must be delivered to the desired site of action. The delivery of inhaled drugs to the lower respiratory tract of mechanically ventilated patients is complicated by deposition of the aerosol particles in the ventilator circuit and endotracheal tube, and the factors governing pulmonary deposition in mechanically ventilated patients are different from those in ambulatory patients. Meticulous adherence to several steps in the technique of aerosol administration is necessary for successful aerosol therapy in mechanically ventilated patients. With a proper technique of administration, an increasing number of inhaled drugs may be administered safely, conveniently, and effectively to mechanically ventilated patients.