Idiopathic thrombocytopenic purpura (ITP) is a disorder that has distinct clinical manifestations in children and adults. In children, ITP is typically abrupt in onset and self-limited in its course. Girls and boys are equally affected. In adults, ITP is typically more indolent in its onset and the course is persistent, often lasting many years or characterized by recurrent exacerbations of disease. Most patients are young women. Intracerebral hemorrhage, while rare, is the most common cause of death in children and adults with ITP. In both children and adults, the ITP is assumed to be caused by antibody-mediated platelet consumption, where Fc receptors on the macrophage bind to antibody-coated platelets resulting in an accelerated platelet destruction. However, tests for antiplatelet antibodies have not yet been established as clinically useful for management decisions. The natural history and long-term prognosis of adults with ITP remain incompletely defined.