Optical coherence tomography (OCT) is a novel technique for two and three-dimensional imaging of tissues at a histological level. The technique is based on optical technology and commercially available fiber-optic components that may be adapted for use in conventional endoscopes or intravascular catheters. OCT is a non-invasive technique, which does not utilize ionizing radiation, and it may within a few seconds provide in vivo images ("optical biopsies") of tissues in cases where excisional biopsy is hazardous or impossible, or when repeated examinations are required. OCT has numerous potential clinical applications, and the technique is currently used in ophthalmology, where it may improve diagnosis and therapeutic control of various eye diseases. Detection and characterization of skin tumors and other dermatological diseases is another area where OCT has tremendous clinical potential. In the field of cardiology, intravascular OCT may be capable to contribute to early diagnosis of vulnerable atherosclerotic lesions. The OCT technique is also being developed in other clinical areas and is expected to become integrated in a range of clinical situations in the future.