Corticosteroids are fundamental to the treatment of asthma, but they should be used as part of an overall plan of treatment that includes reduction in allergen exposure. Each patient should have a tailor-made treatment plan based on a careful assessment of the severity of the disease and aimed at achieving normal airway function, as defined by history, inhalation challenges, and lack of variability of home flow-meter readings. ACSs and SCSs should be regarded as different kinds of drugs, and both drugs are needed in the treatment plan. The ways in which these drugs act in patients with asthma to reduce BHR are largely unknown, but ongoing research, based on biopsy specimens, may well provide important clues about their actions and thus about the causes of asthma.