This chapter discusses disorders of non-neoplastic white blood cells. Neutrophilia refers to an absolute neutrophil count (total leukocyte count per microliter × percent of neutrophils) that is greater than two standard deviations above the mean of normal individuals. Neutrophilia may be associated with infections, drug side effects, tissue necrosis, inflammatory or metabolic disorders and hematopoietic neoplasms (CML, CNL). Neutrophilia is frequently associated with lung and GI malignancies. This chapter covers the neutropenia, functional defects of neutrophils, hereditary conditions associated with abnormal neutrophil morphology, eosinophilia, basophilia, monocytosis, benign lymphocytosis, lymphocytopenia, and hemophagocytic lymphohistiocytosis (HLH). Neutropenia refers to an absolute neutrophil count (total leukocyte count per microliter × percent of neutrophils) that is less than two standard deviations below the normal mean of the population. Lymphocytopenia is defined as a total lymphocyte count less than 1000/μL (1.0 × 109/L) in adults or less than 2000/μL (2.0 × 109/L) in pediatric patients (less than 16 years old).