Platelets are essential in maintaining the delicate balance between the fluidity of blood and the retention of blood within damaged vessels. Not only do platelets form hemostatic plugs that prevent bleeding from vascular defects, but they also set the stage for the coagulation cascade. Hereditary disorders of platelet function are traditionally classified as resulting from abnormalities in platelet adhesion, aggregation or secretion. In reviewing the known canine thrombopathias, we emphasize the importance of each normal platelet function in hemostasis. Special attention is given to a new platelet defect in German Shepherd dogs that is being investigated in the author's laboratory. This defect highlights the importance of a fourth, often overlooked, platelet function called platelet procoagulant activity (PCA). It is this PCA that allows platelets to bridge primary and secondary hemostasis. We also outline a strategy to diagnose a bleeding patient with a primary hemostatic defect and review the treatment options for that patient.