Since their discovery almost two decades ago, interleukin-17-producing CD4⁺ T cells (T_H17 cells) have been implicated in the pathogenesis of multiple autoimmune and inflammatory disorders. In addition, T_H17 cells have been found to play an important role in tissue homeostasis, especially in the intestinal mucosa. Recently, the use of single-cell technologies, along with fate mapping and various mutant mouse models, has led to substantial progress in the understanding of T_H17 cell heterogeneity in tissues and of T_H17 cell plasticity leading to alternative T cell states and differing functions. In this Review, we discuss the heterogeneity of T_H17 cells and the role of this heterogeneity in diverse functions of T_H17 cells from homeostasis to tissue inflammation. In addition, we discuss T_H17 cell plasticity and its incorporation into the current understanding of T cell subsets and alternative views on the role of T_H17 cells in autoimmune and inflammatory diseases.