Interleukin-33 (IL-33) is a new member of IL-1 cytokine family, which may possess dual roles. Functioning as a secreted cytokine, it is involved in the regulating of Th2 immune response and has the role of proinflammatory cytokines. Furthermore, as an intracellular nuclear factor, it locates in the nucleus and regulates transcription. IL-33 is considered to be crucial for the induction of Th2-type cytokine-associated immune responses such as host defense against nematodes and allergic diseases by inducing the production of Th2-type cytokines such as IL-S and IL-13 in Th2 cells, mast cells, basophils and eosinophils. In addition, as a proinflammatory cytokine IL-33 is involved in the induction of non-Th2-type acute and chronic inflammation. In this review, we summarize and discuss the current knowledge regarding the roles of IL-33 and IL-33 receptors in host defense and disease development.