This volume provides a set of reviews dedicated to the biology of Interleukin (IL)-10. It includes chapters on its importance for maintaining immune homeostasis in humans, its role in intestinal immunity and its functions during viral and bacterial infections. In addition, it presents reviews on the mechanisms linking innate microbial recognition to the production of IL-10 and on how IL-10 recognition by its receptor functions. The roles of T and B cells as relevant sources of IL-10 are also discussed, with an emphasis on the clinical opportunities offered by IL-10-producing Tr1 cells for the suppression of unwanted immunity. Finally, the functions of other cytokines of the IL-10 family are presented. Collectively, these articles provide a comprehensive overview of our current knowledge on one of the most important anti-inflammatory cytokines known to date.