CD4⁺ T‐helper cells regulate immunity and inflammation through the acquisition of potential to secrete specific cytokines. The acquisition of cytokine‐secreting potential, in a process termed T‐helper cell differentiation, is a response to multiple environmental signals including the cytokine milieu. The most recently defined subset of T‐helper cells are termed Th9 and are identified by the potent production of interleukin‐9 (IL‐9). Given the pleiotropic functions of IL‐9, Th9 cells might be involved in pathogen immunity and immune‐mediated disease. In this review, I focus on recent developments in understanding the signals that promote Th9 differentiation, the transcription factors that regulate IL‐9 expression, and finally the potential roles for Th9 cells in immunity in vivo.