This review provides an overview of the cellular and molecular characteristics of IL-33 and its receptor, suppression of tumorigenicity 2 (ST2), describes the current evidence on the pro- and anti-inflammatory effects of IL-33/ST2 axis in the course of UC and CD, and discusses the molecular mechanisms underlying the complex association between the IL-33/ST2 signaling axis and IBD pathogenesis. This evidence concerns the gene IL1RL1 and inflammatory bowel disease.