Essential thrombocythemia typically occurs in age groups over 50 years. It is characterized by clonal myeloproliferation with prominent megakaryocytosis. The leading sign is a chronic elevation of abnormal platelets with disturbed function. Diagnosis of essential thrombocythemia is confirmed by the exclusion of other myeloproliferative disorders with a thrombocythemic component and of reactive thrombocytosis. Typical complications are caused by a combined predisposition to thromboembolic complications and hemorrhage. The treatment options fall into two broad categories: antiplatelet agents and platelet-lowering drugs. To date, only subgroups of patients are shown to benefit from these treatment modalities. Survival prognosis is considered close to normal in most patients.