Leukocytes, also known as white blood cells, are hematologic cells important for a host’s immune defense. They comprise several diverse cell types including lymphocytes, neutrophils, monocytes, macrophages, and eosinophils. Each plays a unique and important role in fighting infection, cancer surveillance, and maintaining immune homeostasis. Leukocytes exert their effect and interact with host and foreign cells through the release of cytokines, chemokines, enzymes, and vasoactive substances. Altered number and function of these cells can lead to clinical disorders that range from benign to severe and life-threatening. Here we review the diagnosis, natural history, and treatment of nonmalignant disorders of leukocytes. This review contains 100 references, 6 figures, and 9 tables. Key Words: eosinophilia, hemophagocytic histiocytosis, Langerhans cell histiocytosis, lymphocytopenia, lymphocytosis mastocytosis, neutropenia, neutrophilia