Massive transfusion, historically defined as the replacement by transfusion of 10 units of red cells in 24 hours, is a response to massive and uncontrolled haemorrhage. With more rapid and effective therapy, definitions now identify high risk patients needing rapid issue of blood products because of uncontrolled haemorrhage. Massive transfusion is associated with a number of haemostatic and metabolic complications, most notable being haemostatic changes and dysfunction. These are multifactorial and result of the dilutional effects of blood loss and vascular volume replacement - particularly on “coagulation” proteins and platelets; activation and consumption of coagulation factors secondary to tissue damage and inflammation, or have reduced activity of coagulation factors from prolonged shock, their associated physiological changes of acidosis, hypoxia, hypothermia, and failure to clear activation peptides that act as competitive inhibitors. Management is complex and requires careful and ongoing consideration of a number of physiological relationships and the clinical scenario. It is a dynamic process that requires dynamic monitoring. There is no clear defining threshold for haematocrit, platelet count, or coagulation factor deficiency - the primary concern is maintenance of oxygen and nutrient delivery to tissue and correction of ischaemia. However it is important to establish clinical governance and protocols. Measurement of the haemostatic dysregulation that is associated with this process is not well reflected by conventional coagulation assays and we need better global assessments of a patient’s haemostatic potential with rapid turnaround times. The complexities of this process will be discussed.