Many cells participate in the pathogenesis of asthmatic inflammation. The mast cell is localized at the interface of the internal and external environment within the lung where it may respond to allergens and other exogenous stimuli. The activation of mast cells leads to the release of mediators that contribute to the early phase of asthmatic inflammation. Mast-cell–derived products may also contribute to the late-phase asthmatic response. This review summarizes the developmental biologic features of the mast cell, its receptor-mediated activation, and its range of preformed, newly synthesized, and induced mediators that contribute to asthmatic inflammation. (J Allergy Clin Immunol 2000;105:S527-34.)