Results of studies of the epidemiology, physiology, histopathology, and cell biology of asthma have revised our conception of the disease. Epidemiologic studies have shown asthma to be an important cause of death, suffering, and economic hardship. Physiologic studies have shown that asthma is a chronic illness characterized by persistent bronchial hyperreactivity. Histopathologic studies have shown characteristic changes: epithelial damage, deposition of collagen beneath the basement membrane, eosinophilic and lymphocytic infiltration, and hypertrophy and hyperplasia of goblet cells, submucosal glands, and airway smooth muscle. Studies of the functions of cells in the airway mucosa suggest that asthma may be fundamentally mediated by a difference in the type of lymphocyte predominating in the airway mucosa but may also involve complex interactions among resident and migratory cells. Asthma may thus result from sensitization of a subpopulation of CD4+ lymphocytes, the Th2 subtype, in the airways. These lymphocytes produce a family of cytokines that favor IgE production and the growth and activation of mast cells and eosinophils, arming the airways with the mechanisms of response to subsequent reexposure to the allergen. This conceptual model has stimulated research along lines that will almost certainly lead to powerful new treatments, and it has already put current therapies in a new light, clarifying the role of antinflammatory agents, especially of inhaled corticosteroids. This conceptual model has some limitations: it ignores new evidence on the role of the mast cell in producing cytokines and depends on results of studies of the effects of inhalation of allergen, although most asthma exacerbations are provoked by viral respiratory infection. Preliminary studies suggest that viral infection and allergen inhalation may involve the activation of different pathways, with viral infection activating production of cytokines by airway epithelial cells. Similar study of the mechanisms activated by inhalation of air toxics may provide important clues as to how they might induce or exacerbate asthma.