Bronchial hyperresponsiveness (BHR) and asthma symptoms are linked to chronic inflammation in the airway. Corticosteroids reduce BHR and asthma symptoms and, although their precise mode of action is not yet certain, it is likely that they suppress the allergic inflammatory response. They may act on macrophages, eosinophils, lymphocytes, and airway microvascular leak to inhibit the late response and BHR that follows allergen exposure, whereas chronic administration may also reduce the number of mast cells in airways to decrease the immediate response to allergen and exercise. In view of the safety of administration, inhaled corticosteroids should be considered as first line therapy for chronic asthma, with the aim of suppressing inflammation and reducing the need for bronchodilator therapy.