Current in vitro or ex vivo models of hemostasis and thrombosis fail to recapitulate the hemodynamic conditions and biorheologic phenomena found throughout the vasculature. Microfluidic technology enables physiologic hemodynamics for the study of platelet deposition and coagulation using minimum volumes of human whole blood. This dissertation describes the application of microfluidic assays, the manipulation of surface-patterned procoagulant and sub-endothelial proteins, anti-coagulation, and flow conditions to investigate platelet function and coagulation under flow. First, we demonstrate a novel method to assess the in vivo or in vitro therapeutic efficacy of anti-platelet therapies on platelet aggregates adhering to collagen type I surfaces. We phenotyped individual healthy donor platelet function responses to in vivo or in vitro aspirin, a common antiplatelet therapy over collagen type I surfaces at venous shear rates. Utilizing the same flow assay, we also characterized mechanism-based resistance to aspirin conferred by non-steroidal anti-inflammatory drugs. Furthermore, we have also developed a new model to assess the intrinsic pathway of coagulation under flow on collagen type I surfaces and investigated the role of the intrinsic pathway in recombinant coagulation factor VIIa (rFVIIa) therapeutic efficacy. We then extended this mechanistic investigation of rFVIIa to flow assays where clotting is initiated by collagen and immobilized lipidated tissue factor to evaluate the role of the intrinsic tenase in conjunction with exogenous rFVIIa when surface-triggered extrinsic pathway is present. Finally, we continued to assess coagulopathic patients by first mimicking resuscitation-driven hemodilution, hyperfibrinolysis, and plasmin-inhibitor therapy under flow. We then evaluated downregulation of platelet function in whole blood from trauma patients during the acute phase of trauma-induced coagulopathy. The development of microfluidics, microfabrication, and its applications in hemostasis and thrombosis is essential in advancing our knowledge of clinical and pathological disorders such as myocardial infracts, hemophilia, and deep vein thrombosis. Beyond this work, microfluidic platforms in hemostasis and thrombosis can potentially be used as drug screening platforms for antiplatelet or clotting factor therapies, or a point of care diagnostic test for bleeding and pin-pointing the therapeutic index of novel biopharmaceutics.