The choice of an aerosol delivery system depends on numerous factors such as the drug itself, the characteristics of the aerosol generator, the patient and his or her disease, the physician, and the clinical setting, notably an emergency situation or not. Some rules always apply: an ultrasonic nebulizer should not be used to aerosolize a drug suspension; whenever possible, the same type of aerosol generator should be used for all inhaled medications received by a given patient; for outpatients, education is a major factor to ensure treatment efficacy. When the deposition of the aerosolized drug is aimed at the terminal respiratory units, nebulizers that generate micronic aerosols should be chosen. When the deposition of the aerosolized drug is aimed at the conducting airways, the metered dose inhaler (MDI) is the first choice. However, the MDI is often ill-used, notably in children and elderly people. Therefore, other inhalation devices have been developed: spacers, dry-powder inhalers, breath-actuated MDIs and, more recently, piezo-electric devices. They have been shown to increase lung deposition of drugs in poor coordinators but they all have limitations, which may affect their clinical efficacy. These limitations include the cumbersome dimensions of spacers, the dependency of lung deposition of dry powders on the inspiratory flow rate, the need for reformulation of breath-actuated or not MDIs with CFC-free gases. Nebulization of drugs should be considered only when no portable device is available for the considered drug, or in case of failure of other forms of aerosol administration.