T helper 17 (T_H17) cells have well-described roles in autoimmune disease. Recent evidence suggests that this effector T cell subset is also involved in tumour immunology and may be a target for cancer therapy. In this Review, we summarize recent findings regarding the nature and relevance of T_H17 cells in mouse models of cancer and human disease. We describe the interplay between T_H17 cells and other immune cells in the tumour microenvironment, and we assess both the potential antitumorigenic and pro-tumorigenic activities of T_H17 cells and their associated cytokines. Understanding the nature of T_H17 cell responses in the tumour microenvironment will be important for the design of more efficacious cancer immunotherapies.