Optical coherence tomography (OCT) is a novel biomedical imaging technique that uses low-coherence optical interferometry to obtain micron-scale resolution cross- sectional images of tissue microstructure noninvasively. OCT fills a valuable niche in imaging of tissue structure, providing subsurface imaging with high spatial resolution (on the order of 10 micrometers) and penetration depths of 1 - 2 mm with no contact or matching medium needed between the probe and the tissue. An OCT system for gastrointestinal (GI) endoscopy has been developed using a small-diameter rotary-scanning probe compatible with standard GI endoscopes and capable of imaging in real-time. To date more than 100 volunteers have been imaged during routine upper and lower endoscopic procedures. Results of imaging in normal organs have demonstrated visualization of morphological layers (epithelium, lamina propria, muscularis mucosa, submucosa, muscularis propria) and microscopic structures (glands, villi, crypts, vessels) in all endoscopically accessible GI organs. It has been observed in more than 30 patients that the EOCT appearance of Barrett's mucosa is clearly differentiable from normal gastric or esophageal mucosa. Furthermore, the EOCT appearance of dysplasia and neoplastic lesions, including adenocarcenoma in Barrett's and villous tumor in colon have been observed and are under investigation. Preliminary data indicate the potential of EOCT for routine clinical diagnostics in GI tissues, including early cancer detection and staging and detection of tumor margins.