Aerosols are commonly used in the treatment of patient with pulmonary disease. The clinician must choose the appropriate aerosol delivery device. For both spontaneously breathing and mechanically ventilated patients, the first choice of device is usually the metered dose inhaler. There are important differences among devices in the dosage delivered to the lungs, for the typical prescription, nebulizers delivers more drug to the lungs than metered dose inhalers, which may be particularly important for acutely ill patients. Keywords: Aerosols, Inhaler, Nebulizers, Propellant, Valves.