Naïve CD4+T cells differentiate into various T cell subsets depending on the specific cytokine environment. TH9 cells are less well-characterized than other T cell subsets, and factors that control their development and function have only recently been identified. It is now clear that TH9 cells play critical roles in immune-mediated diseases, including allergic airway, autoimmune and inflammatory bowel diseases, and cancer. Thus, the promotion or suppression of TH9 cell differentiation, transcriptional control and function may provide novel treatments for clinical inflammation, autoimmune diseases and tumors.