CD4+ T cells have been subdivided into different subsets, largely on the basis of the cytokines they produce. These subsets include Th1, Th2 and regulatory T cells. Recently, another population of T cells have been described, namely Th17, which are characterized by their production of IL-17. Two other important cytokines, which are related to each other, are associated with the development of Th cells, namely IL-12 and IL-23. While IL-12 plays a key role in the differentiation of naïve T cells to Th1 cells, IL-23 promotes the expansion of Th17 cells. IL-12 and IL-23 are heterodimers with a shared subunit, p40. They furthermore bind to receptors which have unique and shared subunits. Several previous studies have evaluated the role of IL-12 in inflammatory diseases on the basis of p40. Therefore a reevaluation of the role of IL-12 and Th1 cells in a range of inflammatory conditions has been carried out. This new wave of studies has resulted in the recognition of the role of IL-23 and Th17 cells in inflammatory conditions, such as arthritis and inflammatory bowel disease. There is also the speculation about a possible role in type 1 diabetes.