Interleukin-33 (IL-33) is a novel cytokine and belongs to IL-1 family expressed in a wide range of organs and cells. IL-33 is a dual-functional molecule: as a classical cytokine it induces immune response and it also regulates gene transcription in the nucleus. Altered expression of IL-33 has been observed in various human diseases such as inflammation, autoimmune diseases, and virus infection. The article reviews advances on immune function of IL-33 and its relation to a variety of human diseases.