Cytokines are protein molecules that are produced in response to a variety of stimuli by many different cells and are responsible for coordination of the immune and inflammatory responses. Tumor necrosis factor, interleukins, and interferons are some of the well-characterized cytokines. These cytokines have many diverse functions and act on nearly every tissue and organ system, but are most noted for their effects on the cells of the immune system. The action of each cytokine on its target cell is mediated through binding to specific cell surface receptors; however, several structurally distinct cytokines may exert similar biological effects, even though they bind to different receptors. Cytokines act mainly in a paracrine or autocrine fashion on neighboring cells, but they may also be released into the circulation and act as hormones on distant cells and organs. The host response to infection is usually associated with multiple disturbances in intermediary metabolism. Infection or endotoxin administration stimulates the production of numerous cytokines by host cells, and these cytokines are involved in mediating many of the pathophysiological responses that occur during the course of infection. Recent studies have shown that cytokines also have significant effects on lipid, glucose, and protein metabolism. This review will focus on the metabolic changes that occur during infection and the role of cytokines in mediating these alterations.