Available delivery systems for ambulatory use include the conventional suspension pressurized metered dose inhaler (MDI), the conventional MDI with add-on devices such as spacing chambers and several powder delivery systems. Gas-driven or ultrasonic nebulizers are also available but are generally reserved for in-hospital use or for the treatment of the most severely obstructed patients. It is difficult to select one best system for use in older children, adolescents and adults; all available systems have their deficiencies and these are outlined here. The most widely prescribed device, the MDI, is misused by some patients with claims of up to one-third of clinic patients showing inadequate inhaler technique. In the 1990s, the MDI has also been criticized for liberating chlorofluorocarbons (CFCs). On occasion, the adjuvants are said to precipitate cough. In some countries, the production of generic or second entry inhalers has been accompanied by vexing concerns over the bioequivalence of therapeutic aerosols. The problem of patient coordination with the inhaler is meant to be dealt with by a variety of add-on devices. Spacing chambers and reservoir systems improve drug delivery and treatment efficacy for some patients. Unfortunately, there are relatively few data on patient spacer technique or optimal teaching methods. There is considerable potential for patient misuse of these superficially simple devices. One common problem is that the devices become dirty and worn but are seldom replaced by patients. A more subtle problem is electrostatic drug adherence to the spacing chamber thereby reducing drug delivery.(ABSTRACT TRUNCATED AT 250 WORDS)