Our understanding of the coagulation and fibrinolytic systems (the hemostatic system) in the adult has progressed rapidly in recent years. However, a more complete understanding of the physiology of the hemostatic system in the neonate has lagged behind that of the adult for many reasons. First, the hemostatic system in the newborn exists in a dynamic state and is rapidly evolving toward the adult system. This situation necessitates the generation of not one, but several, reference ranges (or ranges of normal values) in the postnatal period for the various tests and components of the hemostatic system. Also, microtechniques must be used to perform the required assays, both because of the small size of blood samples available and because of the difficulty of obtaining blood from infants. Recently, many of these problems have been resolved, which resulted in a better appreciation of the development of the hemostatic system in the infant. This article reviews our current understanding of the protein components of the coagulation and fibrinolytic systems in the neonate. Reference ranges for normal values of the various tests and components of the hemostatic system have been provided for the premature and full-term infant at birth and during the first 6 months of life.