The pathology of asthma is characterized by mucus plugging of the bronchi and bronchioles, shedding of the respiratory epithelium with the formation of Creola bodies, denudation of the epithelium, the presence of necrotic eosinophilic material beneath the epithelium, a thickened basement membrane zone, bronchial smooth muscle hypertrophy, and goblet cell hyperplasia. In addition, pronounced tissue eosinophilia is present and Charcot-Leyden crystals may be present in sputum. Information linking the eosinophil to the pathophysiology of asthma is presented and the possibility that the eosinophil is a major effector of the inflammation associated with asthma is discussed.