Coagulation testing in children requires accurate age appropriate reference ranges for correct interpretation. This presentation will discuss our evolving concepts of reference ranges in children, and how these particularly apply to the coagulation system. Sources of error will be discussed. The age related variation will be considered in terms of it’s biological significance as well as its significance for diagnostic laboratories and finally for its impact on the use of anticoagulant drugs in children. There remains much more research to be done in the field of developmental haemostasis.