Aerosol delivered through metered-dose inhalers (MDI) offers a potentially convenient way to deliver bronchodilator agents and corticosteroids to the lungs of patients with asthma and COPD. Unfortunately, most patients are unable to coordinate satisfactorily their actuation with inhalation, a problem overcome by using auxiliary MDI aerosol delivery systems. Left to their own judgment, patients often inhale the aerosol with a high inspiratory flow rather than slowly to produce optimal aerosol deposition within the airways. This problem has been corrected by one of the auxiliary MDI aerosol delivery systems (InspirEase) through auditory, visual, and tactile feedback mechanisms. MDI devices release aerosol at a high jet velocity in large particle sizes, depositing most of the aerosol in the oropharynx which can lead to potential systemic absorption of adrenergic agonists with CNS and cardiovascular side effects, oral thrush, and suppression of adrenocortical activity. All the auxiliary MDI aerosol systems promote delivery of small aerosol particles and markedly diminish oropharyngeal impaction. Of all the systems, only InspirEase provides volume and flow feedback controls to ensure an optimal inhalation maneuver. Auxiliary MDI aerosol systems should always be used for aerosolized corticosteroid administration because they minimize oropharyngeal deposition and improve aerosol delivery efficiency.