Leukocytosis, or elevated WBC count, is a commonly encountered laboratory finding. Distinguishing malignant from benign leukocytosis is a critical step in the care of a patient, which initiates a vastly different decision tree. Confirmation of the complete blood cell count and the WBC differential is the first step. Examination of the PB smear is essential to confirming the automated blood cell differential or affirming the manual differential performed on the PB smear. Next is separation of the leukocytosis into a myeloid versus a lymphoid process. Distinguishing a reactive lymphoid proliferation from a lymphoproliferative disorder requires examination of lymphocyte morphology for pleomorphic lymphocytes versus a monomorphic population, with the latter favoring a lymphoproliferative neoplasm. Samples suspicious for lymphoproliferative disorders can be confirmed and characterized by flow cytometry, with molecular studies initiated in select cases; precursor lymphoid neoplasms (lymphoblasts) should trigger a BM examination. Myeloid leukocytosis triggers a differential diagnosis of myeloid leukemoid reactions versus myeloid malignancies. The manual differential is key, along with correct enumeration of blasts and blast equivalents, immature granulocytes, basophils, and eosinophils and identifying dysplasia to identify myeloid malignancies. Confirmation and characterization of myeloid malignancies should be performed with a BM examination and the appropriate ancillary studies. Myeloid leukemoid reactions commonly result from infections and show activated neutrophil changes on morphology; these should prompt evaluation for infection. Other causes of reactive myeloid leukocytoses are also discussed herein.