Inhaled corticosteroids (ICSs) are the gold standard anti-inflammatory therapy for asthma and have been studied using a variety of different clinical trial designs. In long-term comparative studies ICSs are more effective in controlling asthma than beta-agonists or leukotriene antagonists (LTAs). Efficacy has also been shown retrospectively, as patients frequently experience an exacerbation of their asthma upon withdrawal of ICSs, whilst the regular use of low dose ICSs prevents death from asthma. The combination of ICSs with long-acting beta2-agonists (LABAs) is effective for patients with asthma non-responsive to low doses of ICSs, particularly in reducing exacerbations. In shorter term studies a modest dose-response effect of ICSs has been shown for lung function, symptom control and oral corticosteroid use in asthmatic patients. ICSs are also effective in reducing airway hyperresponsiveness (AHR) to various stimuli, as well as reducing exhaled nitric oxide (NO) concentrations and the number and activation state of a wide variety of inflammatory cells. Finally, using allergen challenge models even single doses of ICSs have profound inhibitory effects on the late asthmatic reaction. Since ICSs are the mainstay of asthma management guidelines, it is important that novel therapies should be judged against ICSs in future clinical trials. There are many potential designs for these comparative studies.