Modern advances in the diagnosis and treatment of the leukemias and lymphomas make use of dynamic techniques in immunology, genetics, and molecular biology, and, as a result, morphology represents a smaller proportion of the total diagnostic work-up of these diseases today than heretofore. However, morphology still remains a basic and necessary starting point in the study of these diseases, as is evidenced in the continuing general application of classifications such as the FAB (French-American-British) for the leukemias, and the "Working Formulation" (Working Formulation of Non-Hodgkins Lymphoma for Clinical Usage) for the lymphomas. Although accurate interpretation of cell-type is often possible at the light microscopic level, there are other occasions when electron microscopy may be necessary or useful. Furthermore, a familiarity with the ultrastructure of hematopoietic, lymphoid, and other cells should increase the pathologist's and hematologist's diagnostic acumen and, perhaps, even enhance his or her perspective at the light microscope level.