Mast cells are found beneath the basement membranes, near blood vessels in the submucosa, adjacent to submucous glands, scattered throughout the muscle bundles, in the interalveolar septa and in the bronchial lumen. The evidence that mast cells and mast cell-derived mediators play a role in allergic and non-allergic asthma is discussed. In allergic individuals, inhalation of specific allergens leads to mast cell degranulation and release of mediators. Many of the pathologic features of asthma may be attributed to the effects of mast cell-derived mediators. Their role is clear in allergic asthma and the presence of mast cell derived mediators in the plasma of individuals with exercise-induced and nocturnal asthma suggests involvement in other forms of asthma as well.