Inherited platelet defects bleeding syndromes underlie of varying severity. The Bernard–Soulier syndrome and Glanzmann thrombasthenia are disorders of membrane glycoproteins. In the former, a deficiency of the GPIb-IX-V complex leads to defective platelet adhesion, while in thrombasthenia, platelet aggregation does not occur in the absence of the integrin ´IIbβ3. Defects of primary receptors for stimuli are increasingly being described, and include a defect of a newly cloned Gi-protein-linked, seven transmembrane domain, ADP receptor. These lead to agonist-specific deficiencies in the platelet function response, as do abnormalities in the many intracellular signaling pathways of platelets. Defects affecting secretion from dense bodies and α-granules, of ATP production and generation of procoagulant activity, are also encountered. Some disorders are exclusive to megakaryocytes and platelets, while in others, such as the Chediak–Higashi, Hermansky–Pudlak and Wiskott–Aldrich syndromes; the molecular lesion extends to other cell types. Disorders affecting platelet morphology, the so-called ‘giant platelet’ syndromes should also be considered. In familial thrombocytopenias, platelets are produced in insufficient quantities to assure hemostasis. Platelet disorders are examples of rare diseases; nevertheless they have provided essential information in the elucidation of the molecular basis of platelet function.