Interleukins are a family of proteins that regulate the maturation, diff- entiation, or activation of cells involved in immunity and inflammation, and belong to a broader family termed cytokines. Collectively these proteins are the key orchestrators of host defense and the response to tissue injury. There are currently 23 different interleukins (numbered from IL-1 to IL-23), although the full extent of the interleukin family will only become clear upon analysis of the human genome sequence. Most important, interleukins are central to the pathogenesis of a wide range of diseases that involve an immune com- nent, including such conditions as rheumatoid arthritis, multiple sclerosis, ulcerative colitis, psoriasis, and asthma. Interleukins have also been imp- cated in other conditions, including cancer, migraine, myocardial infarction, and depression. In essence, when cells are activated by interleukins, a program of gene expression is initiated in the target cell that alters the cell’s phenotype, leading to enhanced immune reactivity, inflammation, and/or proliferation. Interleukins are therefore at the core of the cellular basis for many diseases. They are the subject of intense investigation by biomedical researchers and the targeting or use of interleukins in the clinic is proceeding apace. Approaches such as t- geting IL-4 in asthma or IL-1 in joint disease are being pursued, and it is likely that in the next 5-10 years a number of new therapies based on either inhib- ing or administering interleukins will be available.