Barrett's esophagus is a condition in which the normal stratified squamous epithelium is replaced by a specialized metaplastic columnar epithelium. It develops as a consequence of chronic gastroesophageal reflux and predisposes to the development of esophageal adenocarcinoma. Adenocarcinoma develops in Barrett's esophagus by a multistep process in which specialized metaplasia progresses to dysplasia, then to early adenocarcinoma, and eventually to deeply invasive and metastatic disease. This neoplastic progression is associated with a process of genomic instability that generates abnormal clones of cells, some of which have aneuploid or increased G2/tetraploid DNA content. A systematic protocol of endoscopic biopsy can detect Barrett's adenocarcinomas at an early stage, when they may be curable.