White blood cells (WBCs), both granulocytes (e.g., neutrophils, eosinophils, and basophils) and mononuclear cells (e.g., monocytes and lymphocytes), are central to the innate and adaptive immune system, serving to mitigate infectious, traumatic, and malignant insults. Knowledge of normal leukocyte development and function is essential to understanding how their absence and/or dysfunction results in disease. While primary neutrophil disorders have been a long-standing mainstay of hematology training, the increasing recognition of neutropenia and other WBC aberrations as a feature of primary immunodeficiency disorders and autoimmune/immune dysregulation disorders reinforces the need for hematologists to have a broad differential in the evaluation of patients with leukocyte abnormalities. Disorders of white cells are very common in clinical practice. White-cell development and numbers are controlled by a mixture of external stimuli including cytokines, matrix proteins, and accessory cells. Several different white-cell lineages are recognised; each has a role in host defence. Both white-cell deficiency and overproduction can lead to disease. Some forms of inherited white-cell deficiency are potentially treatable with gene therapy