The cytokine interleukin-17 (IL-17) has received considerable attention since the discovery of a distinct CD4⁺ T helper (T_H) cell subset that produces it, known as the T_H17 cell subset. Despite the fact that most of the recent literature describes IL-17 as a T cell-secreted cytokine, much of the IL-17 released during an inflammatory response is produced by innate immune cells. In this Review, we explore the many innate immune cell populations that are an early source of IL-17 in response to stress, injury or pathogens. These early sources have been shown to have a central role in the initiation of IL-17-dependent immune responses, even before the first CD4⁺T cell sees its cognate antigen and initiates the T_H17 cell developmental programme.