Interleukin-17 (IL-17)-mediated immune responses play a crucial role in the mucosal host defence against microbial and fungal pathogens. However, the chronic activation of IL-17-producing T helper cells can cause autoimmune disease. In addition, recent studies have highlighted key roles of innate cell-mediated IL-17 responses in various inflammatory settings. Besides inflammation, there have also been intriguing findings regarding the involvement of IL-17 responses in the pathogenesis of cardiovascular diseases and tumour formation. Here, we discuss the latest discoveries in regulation and function of innate and adaptive IL-17-producing cells.