Because of the problems associated with delivery of aerosols from MDI devices, such as incorrect administration, excessive deposition of aerosol on the oropharyngeal mucosa, and lack of reproducible dosing, several auxiliary MDI delivery systems have been introduced into clinical medicine. All feature some type of holding chamber to store aerosol after actuation of the MDI. The increased residence time for aerosols released from MDI devices into the holding chamber enhances vaporization of propellant to achieve a smaller particle size and most of the impaction loss occurs in the auxiliary device (Table 4) rather than the oropharynx. At low inspiratory flow, these devices prevent excessive aerosol deposition in the oropharynx but at higher flows, deposition from the spacer devices markedly increases (Table 3). The lesser oropharyngeal deposition protects against potential absorption of aerosolized P-adrenergic agonists through the mucosa to produce systemic side effects and prevents oral thrush associated with aerosolized corticosteroids. A mouthpiece provided on all systems enables the aerosol inhalation to be taken with the mouth closed around the mouthpiece. With the exception of one device (InspirEase), all the auxiliary systems have holding chambers opened to the atmosphere with potential for leakage of aerosol from the holding chamber if not coordinated with inhalation.