The discovery that the IL-23-IL-17 immune pathway is involved in many models of autoimmune disease has changed the concept of the role of T-helper cell subsets in the development of autoimmunity. In addition to TH17 cells, IL-17 is also produced by other T cell subsets and innate immune cells; which of these IL-17-producing cells have a role in tissue inflammation, and the timing, location and nature of their role(s), is incompletely understood. The current view is that innate and adaptive immune cells expressing the IL-23 receptor become pathogenic after exposure to IL-23, but further investigation into the role of IL-23 and IL-17 at different stages in the development and progression of chronic (destructive) inflammatory diseases is needed. Rheumatoid arthritis (RA) and spondyloarthritis (SpA) are the two most common forms of chronic immune-mediated inflammatory arthritis, and the IL-23-IL-17 axis is thought to have a critical role in both. This Review discusses the basic mechanisms of these cytokines in RA and SpA on the basis of findings from disease-specific animal models as well as human ex vivo studies. Promising therapeutic applications to modulate this immune pathway are in development or have already been approved. Blockade of IL-17 and/or TH17-cell activity in combination with anti-TNF therapy might be a successful approach to achieving stable remission or even prevention of chronic immune-mediated inflammatory diseases.