IL-10 is an important anti-inflammatory cytokine that plays important roles in controlling inflammatory responses and keeping the immune system in check following activation. Loss of IL-10 function in mice or humans results in the development of inflammatory bowel disease in response to an elevated immune response to the gut flora. IL-10 also acts to prevent excessive inflammation during the course of infection and has been implicated in a variety of autoimmune conditions. In response to inflammatory signals, IL-10 can be produced by a number of immune cells including T cells, B cells, macrophages, and dendritic cells. Distinct mechanisms control the production of IL-10 in these different cells types. In this review, we describe recent studies that have looked at the signaling pathways that regulate IL-10 production in these cells. Given the number of cell types that produce IL-10, it is perhaps not surprising that the in vivo source of IL-10 can vary in different immune models. We also describe how work using conditional IL-10 knockout mice or adoptive transfer of IL-10-deficient cells has begun to further our understanding regarding which specific immune cells are required for IL-10 production in vivo under different conditions.