Optical coherence tomography (OCT) is an intravascular imaging modality, which provides high resolution images up to 10-20 microm. This feature of OCT allows the visualization of specific components of the atherosclerotic plaques in vivo, similar to the histological examinations in vitro. OCT has made a great contribution to the investigation of the pathophysiology of acute coronary syndrome (ACS) in vivo. Furthermore, OCT has a potential not only to detect vulnerable plaques but also to identify vulnerable patients, and may enable us to predict and prevent ACS in the near future.