Thrombocytosis is a common haematological condition encountered worldwide. This chapter outlines a comprehensive approach to its assessment, beginning with a thorough history and physical examination to identify potential underlying causes. Diagnostic tools are highlighted as essential for distinguishing between primary thrombocytosis (such as essential thrombocythemia) and secondary, or reactive, thrombocytosis. Management of reactive thrombocytosis centres on addressing the underlying cause, while cases of essential thrombocythemia—particularly in high-risk patients—may require antiplatelet therapy and cytoreductive agents. Regular monitoring and follow-up are critical to detect complications such as thrombosis or bleeding. Referral to a haematologist is advised for complex cases or when a myeloproliferative disorder is suspected.