Inherited disorders of platelets are characterized by highly variable mucocutaneous bleeding manifestations and excessive hemorrhage following surgical procedures or trauma. They include patients who have decreased platelet counts, abnormal platelet function, or a combination of both. The platelet dysfunction in these patients arises from diverse mechanisms, with most patients having mild to moderate bleeding manifestations. Several molecular and genetic abnormalities have been identified as leading to inherited thrombocytopenias and platelet dysfunction and may also include mutations in hematopoietic transcription factors (e.g., RUNX1) associated with abnormalities in multiple mechanisms involved in platelet production and function. In about half of patients, however, the mechanisms are unknown. In this chapter, clinical and laboratory aspects of inherited platelet disorders are discussed.