Optical coherence tomography (OCT) has become the cornerstone technology in clinical and research imaging in the past two and half decades. OCT performs in vivo, real-time, noncontact scanning and provides cross-sectional and volumetric images with a resolution approaching that of histology. The technology has been used in various medical disciplines, but it is still most profoundly used in the field of ophthalmology where it was initially applied. OCT is continuously evolving with newly developed applications. This chapter will describe the basic principles of OCT techniques, its history, current status, major ophthalmic applications, and research that will determine the future of the technology.