Optical Coherence Tomography (OCT) measures the delay of light scattered from different depths within biological tissue using low coherence interferometry in order to reconstruct the axial reflectivity profile of the sample. With miniature side-looking fiber optic probes, deployed through a narrow, flexible catheter, intravascular OCT investigates the wall of the coronary arteries. The optical probe rotates within a transparent sheath and with a combined pullback motion performs a helical scanning pattern on the luminal surface of the vessel. During acquisition, blood -which is translucent and obscures optical image formation -is temporarily displaced by the infusion of saline or contrast agent through the guide catheter. With current state-of-the-art OCT systems, however, imaging speed is sufficient so that full coronary scans can be registered in just a few seconds. The physics of OCT and of light propagation in biological tissues underpins both the qualitative and quantitative aspects of images as well as subtleties of interpretation of image features and artifacts. The objective of this chapter is to provide a thorough introduction to the physics necessary for the correct and accurate application of intravascular OCT.