The incidence of inflammatory bowel disease (IBD) is steadily in the rise in Western as well as in developing countries paralleling the increase of westernized diets, characterized by high protein and fat as well as excessive sugar intake, with less vegetables and fiber. An interesting hypothesis is that environmental (food-) triggered changes of the intestinal microbiome might cause a proinflammatory state preceding the development of IBD. Indeed, an intact intestinal epithelial barrier assuring a normal bacterial clearance of the intestinal surface is crucial to guarantee intestinal homeostasis. Any factors affecting the epithelial barrier function directly or indirectly may impact on this homeostasis, as well as any changes of the intestinal microbial composition. It is intriguing to learn that some frequently used food components impact on the quality of the intestinal barrier, as well as on the composition of the intestinal microbiome. This highlights the close interaction between living conditions, hygiene, food habits and food quality with the bacterial composition of the intestinal microbiome and the activation status of the intestinal immune system. There is clear evidence that nutritional therapy is highly successful in the treatment of Crohn's disease (CD). Exclusive enteral nutrition is well established as induction therapy of CD. New diets, such as a CD exclusion diet or defined diets (specific carbohydrate diets, FODMAP diet, Paleolithic diet) are being discussed as treatment options for IBD. Well-designed clinical trials in IBD are urgently required to define the precise role of each of these diets in the prevention or management of IBD. Up to now, the role of diet in IBD is highly undermined by lay and anecdotal reports without sufficient scientific proof.