Recent studies have highlighted a role for the alarmin interleukin (IL)-33 in CD4⁺ and CD8⁺ T cell activation and function, and have also revealed important distinctions. The IL-33 receptor ST2 is constitutively and abundantly expressed on T-helper-2 (Th2) and GATA-3⁺ regulatory T cells in a GATA-3- and STAT5-dependent manner. Upon activation, Th1 and cytotoxic T cells express ST2 transiently, driven by T-bet and/or STAT4. We review these findings here, and critically examine evidence indicating that IL-33 enhances the differentiation and functionality of various T cell subsets through positive feedback loops involving lineage-specifying transcription factors. In this context, we discuss how quantitative and qualitative differences in ST2 expression between effector and GATA-3⁺ regulatory T cells may contribute to immune homeostasis, and outline important areas of future inquiry.