Thrombocytosis is a common occurrence in general medical and surgical patients and can have a primary or secondary cause. Investigating a patient with thrombocytosis requires a systematic approach. Secondary thrombocytosis is often self-limiting and not associated with any thromboembolic complication or vasomotor symptoms. Essential thrombocythaemia is a clonal myeloproliferative neoplasm, which is associated with broad clinical sequelae and serious complications such as haemorrhage and thrombosis. Discovery of the JAK2-V6l 7F, MPL and CALR genes has deepened understanding of the pathogenesis of the disorder as well as providing targets for therapy and guidance for prognosis. Subjects with essential thrombocythaemia are risk stratified and managed accordingly. Unless contraindicated, antiplatelet therapy is given to all patients, and cytoreductive therapy is reserved for those who are high risk (i.e. over the age of 60 years or with previous history of a thromboembolic event).