Plasma membrane receptors control macrophage activities such as growth, differentiation and activation, migration, recognition, endocytosis and secretion. They are therefore important in a wide range of physiological and pathological processes including host defence, inflammation and repair, involving all systems of the body including the arterial wall and nervous system. The versatile responsiveness of these cells to various stimuli depends on their ability to express a large repertoire of receptors, some restricted to macrophages and closely related cells, others common to many cell types. This volume contains reviews of the macrophage receptors that are best characterized and deals with aspects of signal transduction and function of the actin cytoskeleton. Our introduction is designed to place these topics in perspective. We summarize features of constitutive and induced mononuclear phagocyte distribution within the body and consider receptor expression and macrophage responses in the context of cell heterogeneity associated with its complex life history. We classify receptors discussed in detail in other chapters, list ligand-binding properties that are not as well defined, and briefly review general features of receptor function in macrophages. An understanding of macrophage receptor biology should bring insights into the contribution of these cells to physiology and disease and result in an improved ability to manipulate activities within the mononuclear phagocyte system.