Hyperresponsiveness is a key abnormality in asthma and deserves recognition both in the clinical presentation of asthma and its pathogenesis. Available evidence strongly suggests that inflammatory processes are important in producing this aspect of disordered airway physiology. Pharmacologic studies have greatly increased our knowledge of mechanisms involved in hyperresponsiveness but fall short of a complete explanation of the phenomenon. It is most likely that hyperresponsiveness, as measured in the laboratory, has many different components and that any single mechanism will be an insufficient explanation.