Crohn's disease and ulcerative colitis are related genetic disorders. Epidemiologic studies suggest that both disorders are caused by a complex interplay of genetic and environmental factors. Genetic linkage studies identify the general chromosomal locations of disease susceptibility genes, and a number of genetic linkages have been reported in inflammatory bowel disease (IBD). Most notable among these linkage regions has been the linkage in the pericentromeric region of chromosome 16, IBD1, among families multiply affected with Crohn's disease. Recent studies have established that at least three coding region variants in the Nod2 gene are responsible for the linkage findings here, and Nod2 therefore represents the first definitively established gene contributing to the pathogenesis of IBD. The implications of these findings for advancing our understanding of Crohn's disease are discussed.