Trauma has a profound impact on public health around the world. Yearly approximately 5 million people die due to traumatic injury, which is 1 out of every 3 severely injured patients. Therefore, improving survival after trauma is a major challenge in which timely therapy is of great importance. In trauma patients, massive haemorrhage is one of the leading causes of mortality. Exsanguination accounts for more than 30% of mortality in trauma patients. The main part of the treatment of massive haemorrhage is to stop the bleeding. However, the development of trauma-induced coagulopathy (TIC) hampers this and exacerbates the bleeding. Therefore, treatment of coagulopathy is a cornerstone in achieving haemostasis and in therapy of bleeding trauma patients. This thesis focusses on knowledge gaps in the field of diagnosis and treatment of TIC in severely injured trauma patients. In order to explore potential diagnostic tools for TIC and to investigate potential strategies to optimise treatment of TIC, the Academic Medical Center of Amsterdam has been participating in the International Trauma Research Network (INTRN) since 2012. The INTRN is a consortium of 6 European Level-1 trauma centres, which received funding from the European Union Framework Programme 7 (FP7) to perform research in the field of coagulopathy after trauma. This thesis is partly established by collaboration with INTRN and by using a large database of trauma patients. The aim of this thesis is to evaluate diagnostic tools for TIC and to investigate which transfusion strategy is associated with the best outcome after trauma. The first part of this thesis focusses on optimising diagnosis of TIC, whereas the second part of this thesis focusses on optimising treatment of TIC.