Intravascular thrombus often occurs when the rupture or fissure of an atherosclerotic plaque exposes procoagulant elements lying below the plaque surface. This process of plaque rupture is the result both of passive mechanical forces and an active cell-mediated mechanism. Recently a number of investigators have developed imaging technologies based on magnetic resonance to characterize atherosclerotic lesions or the use of radioactively labeled platelets to dynamically visualize clot formation and lysis in vivo. These methods may permit early detection of developing thrombus, improve selection of treatment, and allow direct monitoring of the progression of antithrombotic therapy. (Am Heart J 1998;135:S361-S366.)