Celiac disease is defined as a GSE. The small intestinal histological appearance of villous atrophy with crypt hyperplasia, inflammatory cell infiltrate of the lamina propria, and epithelial cell abnormalities is characteristic but not pathognomonic of the disorder. Confirmation of the diagnosis depends on histological improvement when gluten is removed from the diet and deterioration following gluten reintroduction. The pathogenesis of celiac disease appears to require interaction between a number of factors both intrinsic (genetic susceptibility, activation of the immune system) and extrinsic (gluten susceptibility, activation of the immune system) and extrinsic (gluten and possibly other environmental factors). The diagnosis of GSE may be delayed or missed unless the clinician is aware of the broad clinical spectrum of disease presentation. Although celiac disease is widely perceived as a malabsorption syndrome of childhood, the diagnosis is increasingly being made for the first time in adult life. A significant number of patients have no GI symptoms whatsoever. Small intestinal biopsy through the endoscope is the initial and definitive investigation. Most patients show excellent clinical and histological response to a gluten-free diet. The commonest reason for poor response is continuing intentional or inadvertent gluten intake. A minority of patients develop complications, in particular intestinal malignancy, including enteropathy-associated T-cell lymphoma.