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Since their discovery almost two decades ago, interleukin-17-producing CD4 T cells (T17 cells) have been implicated in the pathogenesis of multiple autoimmune and inflammatory disorders. In addition, T17 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 T17 cell heterogeneity in tissues and of T17 cell plasticity leading to alternative T cell states and differing functions. In this Review, we discuss the heterogeneity of T17 cells and the role of this heterogeneity in diverse functions of T17 cells from homeostasis to tissue inflammation. In addition, we discuss T17 cell plasticity and its incorporation into the current understanding of T cell subsets and alternative views on the role of T17 cells in autoimmune and inflammatory diseases.