Abnormalities of leukocyte counts are commonplace in clinical practice with a broad array of etiologies and clinical presentations. While most often these are reactive processes, the broad differential diagnoses and possibility of a clonal process frequently leads to consultation with a hematologist. The diagnostic approach to patients with leukocyte count abnormalities begins with a comprehensive history, physical examination, appropriate laboratory workup, and generally essential is microscopic examination of the blood smear. The following questions focus on frequently encountered nonmalignant leukocyte disorders, with emphasis on abnormalities on neutrophil and eosinophil disorders.