OCT started as a research tool but now is graduating to become an important clinical tool particularly to view the endoluminal structure of the blood vessels. The basic principles of OCT consist of a single mode optic fiber that transmits and receives infra red light waves at a bandwidth of 1250 1350 nm which is the ideal spectral range for imaging biological tissue. The received signals depending on the tissue characters that determines the back scatter attenuation are reconstructed and displayed in cross section. The back scattered optical echoes are processed based on the frequency change frequency domain or time change time domain . As the speed of light is extremely high a reference mirror is used in the time domain OCT to calculate the relative rather than the absolute change. Processing of frequency domain OCT signals has better spatial and temporal resolution. The present generation OCT machines use only FD OCT. As blood is opaque to light it has to be replaced with a transparent medium like the contrast medium during image acquisition. The learning curve for OCT image interpretation is much shorter than IVUS and because of its high resolution the type of plaque can be recognized well and appropriate strategy planned.