Nutritional assessment and dietary advice are fundamental to inflammatory bowel disease (IBD) patient management and all patients should have access to a dietitian. Newly diagnosed patients often think that their pre-illness diet has contributed to the development of their IBD. However, epidemiological evidence to support diet as a risk factor is lacking. How the diet contributes to the gastrointestinal microbiota is interesting, although the role is not yet clearly defined. Nutritional problems in IBD are common. Malnutrition occurs in up to 85% of patients and weight loss affects up to 80% of patients with Crohn's disease and 18–62% of patients with ulcerative colitis. Nutritional deficiencies are prevalent, particularly in relation to anaemia and osteoporosis. Intestinal strictures can be problematic in Crohn's disease and limiting fibrous foods that may cause a mechanical obstruction in the gastrointestinal tract is helpful. Patients often explore dietary exclusion to alleviate symptoms but such changes may be self-directed or inappropriately advised and can lead to further nutritional deficiencies. Some patients experience concurrent functional symptoms (e.g. abdominal bloating, abdominal pain, flatulence and diarrhoea) that can significantly affect quality of life. Recently, a group of poorly absorbed carbohydrates that occur naturally in the diet called fermentable oligo-, di-, mono-saccharides and polyols have been associated with functional symptoms by intestinal bacterial fermentation leading to rapid gas production, and an osmotic effect increasing fluid delivery to the colon. Emerging evidence indicates that a diet low in fermentable oligo-, di-, mono-saccharides and polyols can alleviate functional symptoms in IBD.