Successful aerosol therapy generally depends on the small percentage (typically 10 percent) of the drug dose delivered to the lungs from metered-dose inhalers (MDIs), nebulizers, and dry powder inhalers. Deposition of therapeutic aerosols occurs by inertial impaction (in the oropharynx and large conducting airways) and by gravitational sedimentation (in the small conducting airways and alveoli) and is determined by the mode of inhalation, particle or droplet size, and the degree of airway obstruction. Deposition of metered-dose aerosols in the lungs can be enhanced by using MDIs correctly (aerosol release coordinated with slow, deep inhalation, followed by a period of breath-holding); many patients have poor inhaler technique. Extension devices (spacers and holding chambers) make MDIs easier to use and may increase lung deposition to levels achieved by a correctly used MDI while substantially reducing oropharyngeal deposition. Optimal use of air-driven (jet) nebulizers depends primarily on the choice of nebulizers with relatively small droplet size and on the volume fill and compressed gas flow rate.