The term "vulnerable" in the context of human coronary plaques was originally intended to provide a morphological description consistent with lesions prone to rupture. Coronary thrombosis is now recognized as a diverse process arising from plaque rupture, erosion, or calcified nodules. These findings have prompted the search for more definitive terminology to describe the precursor lesion of rupture, now referred to as "thin-cap fibroatheromas." This review provides a focused discussion of the thin-cap fibroatheroma as a specific cause of acute coronary syndromes. The pathology of the unstable plaque is presented in reference to unstable angina, acute myocardial infarction, and sudden coronary death. The influence of risk factors for coronary artery disease on culprit lesion morphology is also discussed. Finally, the value of coronary calcification, as a predictor of unstable plaques in the clinical setting, is explored.