Cytokines are key mediators of immunity and inflammation. These proteins or glycoproteins act as communication signals between different populations of leukocytes but neither their effects nor their production are restricted to immune cells. In the last few years many new cytokines and their effects have been discovered and it was agreed that when the amino acid sequence of a new cytokine was established it would be assigned the name interleukin (IL), with an added number. The detection of cytokines in disease states promises to provide useful information for diagnostic purposes. The therapeutic utility of cytokines has been explored in many clinical and preclinical studies and in a wide variety of infectious diseases, autoimmune diseases and neoplasias. Since the production of cytokines is modulated by several biological agents such as hormones, prostaglandins and drugs, these may also serve as therapeutic targets for immunomodulation.