Cytokines are peptides used by immune and inflammatory cells to communicate with each other and to control the milieu interieur in which they operate. Recent evidence suggests that they are of immense importance in controlling the local and systemic events of the immune response, inflammation, hemopoiesis, healing, and the systemic response to injury. Many of them can now be measured by immunoassay, and the role of such measurements in the diagnosis and management of disease is actively under investigation. Similarly, the availability of recombinant DNA techniques to produce cytokines in almost unlimited quantities is leading to new and exciting therapeutic applications.