Interleukin-33 (IL-33), a newly described member of the IL-1 family, is expressed by many cell types following pro-inflammatory stimulation and is thought to be released on cell lysis. The IL-33 receptor, consisting of ST2 and IL-1 receptor accessory protein, is also widely expressed, particularly by T helper 2 (T_H2) cells and mast cells. IL-33 is host-protective against helminth infection and reduces atherosclerosis by promoting T_H2-type immune responses. However, IL-33 can also promote the pathogenesis of asthma by expanding T_H2 cells and mediate joint inflammation, atopic dermatitis and anaphylaxis by mast cell activation. Thus IL-33 could be a new target for therapeutic intervention across a range of diseases.