The incidence of oesophageal adenocarcinoma has increased faster than that of any other solid tumour over the last 50 years. The major aetiological factor associated with this tumour is prolonged gastro-oesophageal reflux resulting in metaplastic change in the squamous mucosa to a columnar mucosa (columnar-lined Barrett's oesophagus) with subsequent dysplasia and ultimately neoplastic change to adenocarcinoma. The natural history of this transformation is poorly documented and guidelines have only been developed recently to aid clinicians in their management of columnar-lined oesophagus. This is a retrospective observational cohort study of 2751 patients from 7 United Kingdom centres collated through the UK National Barrett's Oesophagus Registry. It has followed the course of their progress preceding the diagnosis of columnar-lined oesophagus through the course of their clinical follow-up and endoscopic surveillance. The thesis is divided into 5 principal studies examining the natural history of columnar-lined oesophagus and its progression to adenocarcinoma and a sixth examining the validity of the database. The project has examined demographic and epidemiological factors in columnar-lined oesophagus as well as overall rate of progression to adenocarcinoma. The importance of the length of the metaplastic segment and changes in this length and diagnostic histology with reference to malignant risk are examined and discussed. The progress of the metaplastic segment over time is examined with reference to the detection of dysplasia and adenocarcinoma at surveillance. The influence of antireflux treatment (medical and surgical) and symptomatic duration (and type) on the outcome of patients with columnar- lined oesophagus are described. The project has provided a large cohort (the largest in the world with the exception of a pathology report-based database) for examination of the outcome of columnar-lined oesophagus which makes a significant contribution to knowledge on this subject to aid professional bodies in their guidelines and clinicians in their management of this condition.