Infection, ischaemia, physical damage, or specific immunologic sensitivity should be excluded as far as possible before a diagnosis of non-specific inflammatory bowel disease is made. Non-specific inflammations can be subdivided on the basis of macroscopic and microscopic anatomical criteria. Macroscopic structural abnormalities can be recognized by clinical examination, endoscopy, radiology, and inspection of an operation specimen. These complementary methods of data collection combine with microscopic examinations of tissue to separate disorders that differ in prognosis and possible response to treatment. Anatomic classifications do not necessarily imply differences in aetiology and may change with advances in knowledge.