Under normal physiological conditions, eosinophils are present throughout the gastrointestinal tract distal to the squamous oesophagus. Increases in their numbers signify primary and secondary eosinophilic conditions. The rare primary eosinophilic diseases eosinophilic gastroenteritis and eosinophilic colitis affect fewer than ten in 100 000 people, and are characterised by numerous mucosal eosinophils, distributed in sheets and sometimes extending from the mucosa into the submucosa. Pathogenesis of these diseases is poorly understood, but food allergies and intestinal dysbiosis have been implicated. Presentation ranges from vague abdominal symptoms and systemic complaints to, rarely, an acute abdomen with intestinal obstruction. Diagnosis is made from mucosal biopsy samples taken at endoscopy or from surgically resected specimens that demonstrate substantially increased numbers of eosinophils. Eosinophilia secondary to other conditions, such as pathogenic infections, must be excluded. Subtle eosinophilia has also been identified in the duodenum in functional dyspepsia and in the colon in spirochaetosis. Treatment of eosinophilic gastroenteritis and eosinophilic colitis is based on evidence from case reports and small case series, and first-line therapy includes empirical food-elimination diets and single courses of steroids, whereas relapsing or refractory disease might respond to steroid-sparing immunosuppressive agents and biological agents. The progression of disease in eosinophilic gastroenteritis and eosinophilic colitis is variable: a considerable number of patients have just one episode without relapse, whereas others have relapsing-remitting or chronic disease. Primary and secondary eosinophilia in the gastrointestinal tract is increasingly recognised as a clinical conundrum waiting to be solved.