We have used confocal videomicroscopy in real time to delineate the adhesive interactions supporting platelet thrombus formation on biologically relevant surfaces. Type I collagen fibrils exposed to flowing blood adsorb von Willebrand factor (vWF), to which platelets become initially tethered with continuous surface translocation mediated by the membrane glycoprotein Ibα. This step is essential at high wall shear rates to allow subsequent irreversible adhesion and thrombus growth mediated by the integrins α₂β₁ and α_IIbβ₃. On subendothelial matrix, endogenous vWF and adsorbed plasma vWF synergistically initiate platelet recruitment, and α₂β₁ remains key along with α_IIbβ₃ for normal thrombus development at all but low shear rates. Thus, hemodynamic forces and substrate characteristics define the platelet adhesion pathways leading to thrombogenesis.