The macrophage plays an important role in host development and physiology, and in pathogenesis of many infectious, immunologic and degenerative disease processes. It displays marked heterogeneity of phenotype in different tissues, reflecting local interactions with other cell types, and contributes to host homeostasis through a varied repertoire of plasma membrane and secretory molecules. Upon isolation from the body it continues to express special, as well as general, features of cellular organisation and function, which make it a delight to study in cell culture.