Confocal laser endomicroscopy (CLE) has been developed to overcome the limitations of the current endoscopic sampling techniques. CLE allows detailed examination of cellular structures since it provides images very similar to images which a pathologist can see in the microscope. At present, probe based CLE (pCLE) is the only available technology of endomicroscopy. Before optical biopsy, a fluorescein is given intravenously and then fluorescent light coming from a horizontal special focal plane is detected during pCLE of the digestive system.