The inflammatory bowel diseases (IBDs) are a group of heterogeneous disorders characterized by acute and chronic inflammatory changes in the small or large bowel, or in both. Increasing incidence and prevalence figures for IBD both in the developed and developing world indicate that environmental factors are at least as significant in IBD as genetic susceptibility. Of these, diet and the host microbiota are likely to play important but as yet poorly defined roles. The major constituents of a standard "Western" diet may contribute to, or protect against, intestinal inflammation via several mechanisms. These include the effects of insulin resistance and short-chain fatty acids such as butyrate, modification of intestinal permeability, the antiinflammatory role of polyunsaturated fatty acids, and the effect of sulfur compounds from protein on host microbiota. This detailed review critically assesses the evidence for the role of diet in the development of IBD and examines the evidence for obesity as a contributing factor to IBD pathogenesis. Particular attention is focused on methodological issues including suitability of cases and controls, confounders such as smoking, and total energy expenditure.