Current diagnosis of acute leukemia includes traditional morphology and cytochemistry supplemented with immunophenotypic, cytogenetic and molecular biologic analyses. This multiparameter approach has revealed the biological heterogeneity of acute leukemias and has enabled the identification of leukemic syndromes with distinct clinical and biological features. Morphology and cytochemistry are of particular importance for the classification of acute myeloid leukemia, except for certain subtypes such as minimally differentiated acute myeloid leukemia [AML-M0] or acute megakaryoblastic leukemia [AML-M7], requiring additional immunophenotypic or ultrastructural analyses. In acute lymphoblastic leukemia [ALL], immunophenotyping is essential for the diagnosis and lineage assignment [B- and T-lineage ALL] of leukemic blasts. Furthermore, it allows the characterization of the maturation stage and certain subtypes, i.e. ALL with coexpression of myeloid antigens [My+ ALL]. Cytogenetic and molecular analyses of leukemic cells have contributed important informations to the understanding of pathogenetic mechanisms in leukemogenesis and have led to the definition of prognostic risk groups and the development of subtype-specific or risk-adapted therapy strategies.