Flow chamber assays, in which blood is perfused over surfaces of immobilized extracellular matrix proteins, are used to investigate the formation of platelet thrombi and aggregates under shear flow conditions. Elucidating the dynamic response of thrombi/aggregate formation to different coagulation pathway perturbations in vitro has been used to develop an understanding of normal and pathological cardiovascular states. Current microscopy techniques, such as differential interference contrast (DIC) or fluorescent confocal imaging, respectively, do not provide a simple, quantitative understanding of the basic physical features (volume, mass, and density) of platelet thrombi/aggregate structures. The use of two label-free imaging techniques applied, for the first time, to platelet aggregate and thrombus formation are introduced: noninterferometric quantitative phase microscopy, to determine mass, and Hilbert transform DIC microscopy, to perform volume measurements. Together these techniques enable a quantitative biophysical characterization of platelet aggregates and thrombi formed on three surfaces: fibrillar collagen, fibrillar collagen +0.1 nM tissue factor (TF), and fibrillar collagen +1 nM TF. It is demonstrated that label-free imaging techniques provide quantitative insight into the mechanisms by which thrombi and aggregates are formed in response to exposure to different combinations of procoagulant agonists under shear flow.