Cytokines are a group of hormone-like polypeptide mediators that play a variety of regulatory roles in both host defense and normal and abnormal homeostatic mechanisms. They may he produced by diverse cell types and exert their function on a variety of cells. Their effects (which may be suppressive or enhancing) are on cellular proliferation, differentiation, activation, and motility. In addition, cytokines can exert cytodestructive effects on infectious agents or tumor cells, either directly or by activating cells with cytodestructive potential. Any given cytokine may have many different biologic effects. However, two different cytokines may have similar or identical activities. Cytokines may be classified on the basis of their cell of origin, their spectrum of activity, the category of activity they influence, the cells that are their targets, or on specific features of their ligand-receptor interaction. The mode of action of many of the cytokines involves typical signal transduction events such as protein phosphorylation, and to date there is only limited understanding of the mechanisms that lead to one activity over another when a specific cytokine is involved in a specific biologic reaction. Nevertheless, elucidation of their role in other pathologic processes has provided insight into autoimmune and allergic diseases, as well as a variety of systemic disorders. Because of their broad spectrum of activity, cytokines have been used in a variety of therapeutic settings involving both infectious diseases and neoplasia. As the number of known cytokines continues to grow, it will be increasingly difficult for the non-"cytokinologist" to follow the exponentially expanding literature. Hopefully, this brief review will provide an overview that can serve as a framework for the understanding of this important area of biology and pathology.