Barrett's oesophagus is a condition where the distal oesophagus is lined with columnar epithelium. For the diagnosis intestinal metaplasia has to be found and a minimal length of 3 cm is required for the columnar-lined segment. It is well known that patients with the traditional Barrett's oesophagus have a higher risk of developing adenocarcinomas in the metaplastic segment. The clinical significance of shorter metaplastic segments and intestinal metaplasia at the gastro-oesophageal junction is much less certain. Different definitions are in use and the increasing number of studies dealing with these mucosal changes therefore may create confusion. In this article, the current knowledge concerning epidemiology, pathophysiology, clinical associations, diagnostic possibilities including endoscopy, pathology, measurement of differences in electrical potential, and treatment of columnar-lined oesophagus of any length and of intestinal metaplasia at the gastro-oesophageal junction is reviewed. Furthermore, the need for a consistent definition is emphasized.