This chapter focuses on platelets disorders and bleeding disorders. Platelets are formed in the bone marrow by a process of fragmentation of the cytoplasm of megakaryocytes. Platelet production is under the control of thrombopoietin. The main functions of platelets are in haemostasis which includes two closely linked processes which are primary haemostasis and secondary haemostasis. Bleeding disorders or haemorrhagic diatheses are a group of disorders characterized by defective haemostasis with abnormal bleeding. The tests used to investigate haemostatic abnormalities are investigation of disordered vascular haemostasis, investigation of platelets and platelet function, and investigation of blood coagulation. Vascular bleeding disorders, also called non-thrombocytopenic purpuras or vascular purpuras, are normally mild and characterised by petechiae, purpuras or ecchymoses confined to the skin and mucous membranes. Disorders of platelets produce bleeding disorders by one of the following 3 mechanisms: due to reduction in the number of platelets, due to rise in platelet count i. e. thrombocytosis and due to defective platelet functions. Idiopathic thrombocytopenic purpura or immune thrombocytopenic purpura (ITP), is characterised by immunologic destruction of platelets and normal or increased megakaryocytes in the bone marrow.