Investigation of platelet function disorders in infants and small children requires the collaborative efforts of clinicians and clinical laboratories. A detailed personal, family, and medication history, and a search for additional clinical phenomena may help to direct diagnostic laboratory investigations. Testing for these disorders in young children presents several challenges: the requirement of relatively large volumes of blood, lack of standardization, and the absence of well-established age-specific reference ranges. Neonates show the most notable differences in platelet function compared to older children and adults; the decreased platelet activation responses persist for the first 2 to 4 weeks after delivery. Small studies of normal postneonatal pediatric populations have provided data on platelet function assays, including bleeding times, PFA-100 closure times, thromboelastography, aggregation, secretion, and flow cytometry. The majority of these studies, comparing normal children with adults, found only minor differences in platelet responses measured by these assays.