Summary. Platelet aggregation and thrombus formation at sites of atherosclerotic plaque rupture is a dynamic process that can lead to intermittent or permanent obstruction to blood flow, resulting in ischemic tissue injury and organ dysfunction. There is a growing body of evidence suggesting that the dynamics of platelet aggregation and initial thrombus development are regulated by two distinct, complementary processes, involving: (i) rheological (biomechanical) and (ii) soluble-agonist-dependent mechanisms. Rheological-dependent platelet aggregation occurs between discoid platelets and requires the biomechanical adhesive and signaling function (mechanotransduction) of the major platelet adhesion receptors, GPIb and integrin αIIbβ3. Soluble agonists further potentiate platelet activation, stimulating global platelet shape change and degranulation, and play a major role in stabilizing formed aggregates. Unraveling the dynamics of platelet aggregation and thrombus formation in vivo requires consideration of the cooperative interplay between rheological- and soluble agonist-dependent platelet aggregation mechanisms.