Interleukin-10 is the most potent anti-inflammatory cytokine yet identified. It has multiple actions affecting the innate immune system as well as humoral and cellular immune responses. It occupies a pivotal role in the regulation of the immune response to microbial pathogens in health and disease. Knowledge gained in the molecular biology of IL-10 and its complex immune effects in experimental infection models are leading to new insights into therapeutic manipulation of IL-10 in patients with systemic inflammatory diseases.