Recently, substantial advances in the understanding of the molecular pathogenesis of inflammatory 
bowel disease (IBD) have been made owing to three related lines of investigation. First, IBD has 
been found to be the most tractable of complex disorders for discovering susceptibility genes, and 
these have shown the importance of epithelial barrier function, and innate and adaptive immunity in 
disease pathogenesis. Second, efforts directed towards the identification of environmental factors 
implicate commensal bacteria (or their products), rather than conventional pathogens, as drivers of 
dysregulated immunity and IBD. Third, murine models, which exhibit many of the features of 
ulcerative colitis and seem to be bacteria-driven, have helped unravel the pathogenesis/mucosal 
immunopathology of IBD.