The interleukin 17 (IL-17) family of cytokines contains 6 structurally related cytokines, IL-17A through IL-17F. IL-17A, the prototypical member of this family, just passed the 25 th anniversary of its discovery. While less is known about IL-17B-F, IL-17A (commonly known as IL-17) has received much attention for its pro-inflammatory role in autoimmune disease. Over the past decade, however, it has become clear that the functions of IL-17 are far more nuanced than simply turning on inflammation. Accumulating evidence indicates that IL-17 has important context- and tissue-dependent roles in maintaining health during response to injury, physiological stress and infection. Here, we discuss the functions of the IL-17 family, with a focus on the balance between the pathogenic and protective roles of IL-17 in cancer and autoimmune disease, including results of therapeutic blockade and novel aspects of IL-17 signal transduction regulation. The IL-17 cytokine family is relatively poorly understood, apart from the prototypical, founding member, IL-17A, which has achieved notoriety for its role in autoimmunity. In this review, McGeachy, Cua and Gaffen discuss the pathogenic and protective roles of the IL-17 family in health, inflammation, injury, microbial regulation and cancer.