Hyperleukocytosis is defined as a white blood cell count greater than 100.000/μL in patients affected by acute or chronic leukemias. Hyperleukocytosis is more common in acute leukemias than in chronic leukemias. Risk factors include younger age, acute myeloid leukemia, the microgranular variant of acute promyelocytic leukemia, acute lymphoblastic leukemia and some cytogenetic abnormalities. Although it can affect any organ system, symptoms usually arise from involvement of the cerebral, pulmonary and renal microvasculature. The term "leukostasis" refers to 'symptomatic hyperleukocytosis' which is a medical emergency that needs prompt recognition and initiation of therapy to prevent renal and respiratory failure or intracranial haemorrhage. The underlying mechanisms of hyperleukocytosis and leukostasis are poorly understood. The management of hyperleukocytosis and leukostasis involves supportive measures and reducing the number of circulating leukemic blast cells by induction chemotherapy, hydroxyurea, low-dose chemotherapy, and leukapheresis. The measures such as hydroxyurea, low-dose chemotherapy, and leukapheresis shouldn't be considered to correct the laboratory abnormalities in patients with hyperleukocytosis who have no signs or symptoms. Also, neither hydroxyurea nore leukapheresis is able to show benefit on short and long term outcomes in patients with symptomatic hyperleukocytosis. The optimal management of symptomatic hyperleukocytosis is still uncertain, and there are no randomized studies demonstrating one is superior to each other. Therefore, it is recommended that intensive chemotherapy should be implemented as quickly as possible in treatment-eligible patients, in parallel with supportive measures for DIC and TLS.