Interleukin-10 (IL-10) is regarded as an immune suppressant cytokine. This reputation is due to the experimental observation that IL-10 decreases the function of antigen presenting cells and T helper 1 type immune responses. Surprisingly, however, IL-10 has potent anti-cancer effects since most experimental models demonstrate immune-mediated anti-tumor effects whether IL-10 is locally (tumor transfectant models) or systemically (transgenic models, administration of soluble cytokine) provided. In addition, recent work has suggested that polymorphisms of the promoter region of IL-10 may segregate propulsions in high and low producers. Surprisingly, high IL-10 producers may be more predisposed to acquire cancer and other diseases. Finally, human observations demonstrate that systemic administration of IL-10 to normal volunteers is associated with very little toxicity and pro-inflammatory properties mediated through activation of effector cells of the innate immune response. Thus, the role that interleukin-10 plays in vivo in physiological or pathological conditions remains controversial. Thus, there exists a compelling need to summarize in a book the state of the science of this important cytokine. With the assistance of several experts in this field, Interleukin-10 has attempted this endeavor.