Inhaled glucocorticosteroids have now become first-line therapy for the treatment of chronic asthma in many countries. They are the most effective asthma therapy currently available, and numerous studies have documented their long-term efficacy in asthma control in adults and in children. Inhaled steroids suppress inflammation in asthmatic airways, although their precise molecular mechanism of action is not yet certain. It is likely that steroids affect the transcription of several steroid-responsive genes, and, of particular importance, they may inhibit cytokine gene transcription and cytokine effects, thereby reducing the chronic inflammation in asthmatic airways. Inhaled steroids are now used at a much earlier stage in asthma therapy, and there is a strong argument for their early introduction in both adults and children to prevent asthma morbidity and mortality and possibly the structural changes resulting from uncontrolled chronic inflammation, which may lead to irreversible airflow obstruction in some patients. Of paramount importance is the question of safety as inhaled steroids are likely to be required for a long time. Local side effects caused by oropharyngeal deposition of the inhaled steroid may be reduced by the use of spacer devices and mouthwashing. Systemic side effects caused by gastrointestinal absorption of the fraction deposited in the oropharynx may also be reduced by these devices. There are differences in the systemic bioavailability of the different inhaled steroids currently in use, and inhaled steroids with the lowest bioavailability should be chosen when high doses of inhaled steroids are required for asthma control. Systemic side effects are usually observed only when daily doses of > 800 micrograms are inhaled, and whether effects on very sensitive biochemical indices are relevant to long-term deleterious effects is not yet certain. There is now overwhelming evidence that the doses of inhaled steroids required to control asthma in the majority of adults and children are safe and without systemic side effects. It is important to control asthma with the minimum dose of inhaled steroids possible, however. In the future it may be possible to develop inhaled steroids with even fewer systemic effects if the fraction absorbed from the respiratory tract can be rapidly metabolized in the bloodstream.