Failure to deliver drug effectively to the lungs is a common cause of referrals with "difficult asthma". This may be due to poor regime compliance or poor device compliance (lack of competence and/or contrivance). The former is the more difficult to address. The latter can be addressed providing the healthcare professional is aware of the principles underlying aerosol delivery and aspects of patient behaviour. Unlike the gastrointestinal tract, the airways have evolved to exclude foreign material. A narrow window of opportunity exists with particles in the size range 1-7 micro m having a relatively high probability of depositing within the airways once inhaled. Current delivery systems are able to generate particles in this size range but they all have associated problems that may significantly impair the effectiveness of drug delivery to the lungs. While there is little evidence that compliance with inhaled therapy is significantly worse than with oral therapy, there is evidence that lack of competence (the inability to use a device effectively) or contrivance (knowing how to use a device effectively but contriving to use it ineffectively) are major additional impediments to effective therapy. Ensuring that the delivery system is acceptable to the patient is particularly important in the very young. The device chosen with a patient should be one that can and will be used effectively. For inhaled steroids, the choice of device/drug combination should be limited to those with a good therapeutic index (i.e. the majority of any systemic exposure should result from the lung dose). The best guide to determining the appropriate dose of inhaled steroid is to use the lowest effective dose.