The determination of the white blood cell count and differential count is important in the medical examination of a patient yet the basic mechanisms guiding leukocyte turnover and dynamics remain largely undefined. This is more than an academic problem since a better understanding of the life of the leukocyte is important in the treatment of leukemia and in the achievement of selective toxicity to the neoplastic cell in the therapy of patients with cancer. The height of the white blood cell count is affected by numerous factors and once leukocytes leave the vascular tree there is no simple way of determining their ultimate fate. However, in spite of many technical problems, sound data have been accumulated concerning leukocyte production, release from the bone marrow, intravascular life span, and distribution. In this paper, the significant information dealing with leukocyte physiology is reviewed with particular emphasis on the granulocyte.