Considerable progress has been made in the last decade in studies of the genetics of the inflammatory bowel diseases, Crohn's disease and ulcerative colitis. Epidemiological data, notably concordance rates in twin pairs and sibling pairs, have provided strong evidence for the importance of the genetic contribution, particularly in Crohn's disease. These observations provided the catalyst for laboratory-based studies of the molecular genetics of Crohn's disease and ulcerative colitis around the world. The complementary strategies of genome-wide scanning and candidate gene-directed studies have led to the identification of a number of genetic markers which appear to predict disease susceptibility and behaviour. The identification of the IBD1 gene on chromosome 16 as NOD-2 is unquestionably an important scientific discovery. Although many issues with respect to gene function and expression remain to be resolved there is great optimism that important clinical applications will directly result.