Inflammatory bowel diseases are both environmental and genetic illnesses. More than one hundred genes or loci involved in the regulation of innate or acquired immune response as well as intestinal mucosa homeostasis have been identified. Environmental studies have been less numerous up to now and only smoking and appendectomy have been validated, as protector for ulcerative colitis, while smoking is clearly associated with an increased risk and more severe forms of Crohn's disease. An important role is also currently suspected for the intestinal flora and the dysbiosis described in inflammatory bowel disease could contribute to the triggering or the persistence of the inflammation. New therapeutic strategies are currently studied, particularly aiming at targeting immune, inflammatory or homeostatic pathways corresponding to the predisposing gene variants.