The allergen challenge has evolved, in less than 150 years, from a crude tool used to document the etiology of allergen-induced disease to a well-controlled tool used today to investigate the pathophysiology and pharmacotherapy of asthma. Highlights of the authors’ involvement with the allergen challenge include confirmation of the immunoglobulin E-dependence of the late asthmatic response, importance of (nonallergic) airway hyper-responsiveness as a determinant of the airway response to allergen, identification of allergen-induced increase in airway hyper-responsiveness, documentation of beta2-agonist-induced increase in airway response to allergen (including eosinophilic inflammation), advances in understanding the pathophysiology and kinetics of allergen-induced airway responses, and development of a muticentre clinical trial group devoted to using the allergen challenge for investigating promising new therapeutic strategies for asthma.