White blood cells or leukocytes, are cells of the immune system involved in defending the body against both infectious disease and foreign materials. The white cell series encompasses those cells that are distinguished by their granules and those that are agranular. There are three types of granulocytes: Neutrophils, basophils, and eosinophils, which are named according to their staining properties. Agranulocytes (mononuclear leukocytes) are leukocytes characterized by the apparent absence of granules in their cytoplasm. This chapter discusses the clinical application of leucocytosis and the conditions causing such abnormalities. An elaborate description of stages of maturation of leucocytes and the diseases involving their formation and maturation, including Leukemic and leukemoid reactions is highlighted.