Infonnation about eosinophils has rapidly expanded over the last few years. This interest has been the result of increasing evidence for the eosinophil's role in the pathophysiological processes of asthma and the hope that modulation of its function would lead to better therapies for the disease. Evidence for the eosinophil's involvement in the airway inflammation seen in asthma is from a variety of studies. The presence and activation of these cells have been demonstrated in bronchial biopsy specimens, in bronchoalveolar lavage (BAL) fluids, and in peripheral blood specimens from patients with mild to moderate asthma (1 ). Furthennore, activation of eosinophils leads to the release of their granule proteins: major basic protein (MBP), eosinophil peroxidase (EPO), eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EON). All four can function as cationic toxins and as cytostimulants. Thus, these proteins have the ability to cause local tissue damage, thereby contributing to the pathogenesis of local inflammation; in fact, their levels have been correlated with severity of clinical symptoms of asthma (2). In this chapter, we will review material on the properties and biological functions of the eosinophil and its granule proteins and relate this infonnation to current studies of the role of the eosinophil in asthma.