Basic information has been presented concerning leukocyte (neutrophil, monocyte, lymphocyte, eosinophil, and basophil) function, production, kinetics, and response to various physiological and disease states. Using this information, veterinary practitioners should be able to interpret leukogram data from sick and healthy dogs and cats. Specifically, characteristic leukogram patterns such as physiological leukocytosis, corticosteroid-associated changes, and the presence of infection or severe inflammation should be recognized. In addition, interpretation of individual leukocyte responses should be possible. Several tables have been provided to assist in constructing a differential diagnosis to explain increases or decreases in absolute leukocyte numbers that lie outside of expected reference intervals.