The modes of evolution of atheromatous plaques are now better understood. The process may start very early and continue for several decades. The severity of atherosclerosis lies in the results of phases of rapid progression involving a thrombotic mechanism. An initial mechanical accident (rupture or erosion of the plaque) is followed by a phenomenon of arterial thrombosis of variable severity. The vulnerability of an atheromatous plaque (defined as its potential for acute complication) is related to: 1) the risk of rupture, 2) the risk of thrombosis in case of rupture. A number of intrinsic properties influence the degree of plaque vulnerability. A thorough understanding of these mechanisms is essential for the development of new treatments to improve the prognosis of coronary patients.