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Context.—Essential thrombocythemia (ET) is a chronic myeloproliferative disorder (CMPD) characterized predominately by thrombocytosis and abnormal megakaryocyte proliferation. The current diagnostic criteria require a combination of clinical, histologic, and cytogenetic data. The diagnosis relies largely on exclusion of other causes of thrombocytosis. Objective.—Describe historical, clinical, and laboratory features of ET in order to understand, clarify, and more accurately diagnose this entity. Data Sources.—Review contemporary and historical literature on ET and other causes of thrombocytosis. Conclusions.—ET is a relatively indolent and often asymptomatic CMPD that is characterized primarily by a sustained elevation in platelets ≥600 × 103/μL (≥600 × 109/L), proliferating enlarged and hyperlobated megakaryocytes, and minimal to absent bone marrow fibrosis. Significant changes and revisions to the diagnostic requirements and criteria for ET have occurred during the last 30 years. Recently, a mutation in the Janus kinase 2 (JAK2) gene has been found in a significant number of cases of ET and other CMPDs. In up to 57% of ET cases, a mutation in the JAK2 gene can be detected. In the absence of a JAK2 mutation and features of another CMPD, the diagnosis of ET remains a diagnosis of exclusion after other causes of thrombocytosis have been excluded.