Platelets are essential for primary hemostasis, and abnormalities in their count or function can cause significant bleeding or thrombosis. Acquired platelet disorders are much more common than inherited ones and can happen in various situations, such as drug exposure, autoimmune diseases, systemic illnesses, and critical conditions. This chapter examines the current understanding of drug-induced and immune-mediated platelet defects, the diseases characterized by increased platelet counts, their clinical manifestations, and treatment approaches.