Interleukin-17 (IL-17) is a cytokine which elicits protection against extracellular bacterial and fungal infections and which plays important roles in inflammation. However, when produced in excess, it contributes to chronic inflammation associated with many inflammatory and autoimmune disorders. This has made IL-17 an attractive therapeutic target. The present review describes the structure of the IL-17 family, the IL-17 receptor complex, and the cells producing IL-17. The contributions of IL-17 to disease as well as new IL-17-based treatment options are discussed. Finally, the results of IL-17 or IL-17 receptor inhibitors in clinical trials are detailed. With a fruitful outlook, drug registration has now been granted for psoriasis psoriatic arthritis and ankylosing spondylitis, and also bears great potential in a growing number of conditions.