Group 2 innate lymphoid cells (ILC2s) are activated by diverse endogenous signals. One such signal, IL-33, is released by necrotic cell death and therefore signals tissue damage. The mechanisms that induce the other signals remain poorly understood, but we propose that they provide ILC2s with information about disrupted tissue homeostasis. When activated, ILC2s secrete IL-5, IL-13, IL-9, and perhaps IL-4. IL-9 enhances activation via an autocrine loop. Abbreviations: thymic stromal lymphopoietin (TSLP); vasoactive intestinal peptide (VIP); prostaglandin D2 (PGD2); cysteinyl leukotriene C4/D4 (LTC4/LTD4); death receptor 3 (DR3); chemoattractant receptor-homologous expressed on Th2 cells (CRTH2); cysteinyl leukotriene receptor 1 (CYSLTR1); vasoactive intestinal peptide receptor 2 (VPAC2).