Increasing evidence has accumulated to suggest that eosinophils play a key role in the pathogenesis of asthma and other pulmonary diseases by damaging infiltrated bronchial tissue and lung parenchyma. The first part of this review on eosinophils describes the cellular characteristics and properties of the cell, which help in understanding its role in disease. The article focuses on origin, maturation and differentiation of the eosinophil, its morphological and phenotypical properties, as well as its preformed and newly generated mediators of inflammation. The cause and putative significance of eosinophil heterogeneity in respect to function and density will also be discussed. In addition, the naturally occurring mediators through which eosinophils are activated and communicate with other inflammatory cells are outlined. The first part closes with new aspects of eosinophil recruitment from the circulation into perivascular tissue, including nonselective and putative selective adhesion mechanisms and chemotaxis.