Inhaled corticosteroids are widely recognized as a highly effective treatment in asthma, by improving pulmonary functions and symptoms, and reducing exacerbations, hospitalizations, and mortality. Inhaled corticosteroid compounds have very different pharmacokinetic and pharmacodynamic properties that are likely to affect their therapeutic effects. The time to onset and the time to maximal response of each inhaled corticosteroid have not been totally defined. Some studies indicate that the onset of action of inhaled corticosteroids occurs within 1 day of beginning therapy. However, there is evidence to suggest that inhaled corticosteroids have early therapeutic effects (< 3 h) in acute severe asthma. In nocturnal worsening of asthma, the use of inhaled corticosteroids as chronotherapy has been shown to reduce the nocturnal worsening of lung function over a period of 12 hours. Chronically, the airway response to inhaled corticosteroids could increase gradually over several weeks and sometimes months. The knowledge of when inhaled corticosteroids begin to exert an effect and the necessary time for the maximal response, although is not totally defined, is useful to determine the optimal duration of a trial with inhaled steroids before patient’s re-evaluation and to inform patients about how quickly they can expect to obtain clinical benefit.