Trauma-induced coagulopathy is a central cause of preventable deaths from hemorrhage after injury. The contribution and impact of altered post injury platelet biology on trauma-induced coagulopathy is not well understood despite the pivotal contribution of platelets to normal coagulation and endothelial integrity. The central hypothesis for this study is that severe injury and shock drive altered platelet activation, platelet aggregation, and platelet-endothelial interactions that are associated with increased rates of transfusion, organ failure, and mortality. This study will investigate these causal pathways, mechanisms, and associated outcomes in a prospective observational trauma cohort through collection of biospecimens and detailed clinical data.