Interleukin 33 (IL33) is a recently described member of the IL1 superfamily of cytokines. Originally defined on the basis of T-cell subset differentiation, IL33 is now recognised to mediate a wider role in regulating components of the innate immune response also, particularly via mast cell activation. In this paper the basic biology of IL33 is described together with that of its cognate receptor, ST2L, and the existing knowledge base for its potential role in mediating human pathology across a range of diseases is defined.