Defects in primary hemostasis typically lead to a bleeding diathesis and include several disorders, such as von Willebrand disease and various platelet function defects that may arise because of failure of membrane receptors, deficiencies in internal storage organelles, and/or signal transduction problems. The laboratory identification of a primary hemostasis defect including platelet function defects now involves a multi-step process. These processes begin with a full blood count to assess platelet count and size, progressing, if necessary, to platelet aggregation studies, and leading to further specific testing including flow cytometry, electron micro-scopy and/or genetic testing. This review outlines the sequential process of platelet function investigations, and discusses each of the essential components in some detail.