Barrett's oesophagus is a metaplastic change, such that the normal squamous epithelial lining of the oesophagus is replaced by specialised columnar lined epithelium. Barrett's oesophagus is clinically significant and has a high health economic impact as it is associated with heightened risk of progression to oesophageal adenocarcinoma. This review discusses the pathogenesis of Barrett's oesophagus with an emphasis on the underlying molecular events.