Interleukin (IL)‐33 is a key cytokine involved in type 2 immunity and allergic airway diseases. Abundantly expressed in lung epithelial cells, IL‐33 plays critical roles in both innate and adaptive immune responses in mucosal organs. In innate immunity, IL‐33 and group 2 innate lymphoid cells (ILC2s) provide an essential axis for rapid immune responses and tissue homeostasis. In adaptive immunity, IL‐33 interacts with dendritic cells, Th2 cells, follicular T cells, and regulatory T cells, where IL‐33 influences the development of chronic airway inflammation and tissue remodeling. The clinical findings that both the IL‐33 and ILC2 levels are elevated in patients with allergic airway diseases suggest that IL‐33 plays an important role in the pathogenesis of these diseases. IL‐33 and ILC2 may also serve as biomarkers for disease classification and to monitor the progression of diseases. In this article, we reviewed the current knowledge of the biology of IL‐33 and discussed the roles of the IL‐33 in regulating airway immune responses and allergic airway diseases.