Chronic lymphoproliferative disorders (CLPDs) are derived from the clonal proliferation of cytologically and immunophenotypically mature B or T cells. Although overlap is prominent, the CLPDs can generally be segregated into leukemias (predominant blood and bone marrow manifestations) and lymphomas (predominant extramedullary manifestations). This review discusses the leukemic processes, with emphasis placed on the blood and bone marrow features of these disorders. Many distinct types of B-cell and T-cell CLPDs have been described, and the classification of CLPDs requires the integration of morphologic features, immunophenotype, and clinical information; genotypic analyses might provide prognostic and biologic information. The various classification systems for the CLPDs will be presented, as well as a discussion of specific disease types, emphasizing those most commonly encountered in clinical practice.