The most common cause of acute coronary syndrome (ACS) is vulnerable plaque rupture. Other less common causes are attenuated plaques, erosions, and calcific nodules. We review the current intracoronary imaging techniques—grayscale intravascular ultrasound (IVUS), virtual histology (VH)-IVUS, optical coherence tomography (OCT), and near-infrared spectroscopy (NIRS)—and their ability to identify the causes of ACS and, potentially, improve the outcomes in ACS patients. Grayscale IVUS gives information about vessel and lesion morphology, but it is limited in terms of plaque composition and near-field resolution. VH-IVUS improves on the ability of grayscale IVUS to assess plaque composition, but it does not improve on resolution. OCT provides structural details with high resolution and can assess plaque composition, but is limited by a low penetration into tissue and the interference of light by blood. NIRS detects lipid core plaques, but does not give structural information. Each of these techniques offers important, but incomplete information when used individually. There are early, but real attempts to combine these technologies in a single catheter and system.