The etiology of inflammatory bowel diseases, Crohn's disease and ulcerative colitis, is uncertain. Studies of specific environmental factors and immune dysfunction have provided limited insight into disease pathogenesis. There is ample evidence that these diseases are in part the result of genetic predisposition. The early search for candidate genes focused on genes involved in the regulation of immune function. Recent genome-wide searches reported several susceptibility loci for Crohn's disease and ulcerative colitis. The recent identification of the IBD1 gene (NOD2) with mutations that are associated with susceptibility to Crohn's disease will have a major impact on the understanding of the genetics of this disease.