Monitoring of residual leukemic cells, so called minimal residual disease (MRD) in acute leukemias is of great importance in clinical practice. Especially evaluation of early response to treatment has a important prognostic value, and modifies the therapy schedule in a nowadays used treatment protocols. Methods used for monitoring of MRD consist of molecular techniques, which detect chromosomal aberrations or immunoglobulins and lymphocyte T receptor genes rearrangements, as well as method of flow cytometry, which detect immunophenotype typical for the leucemic clone. The paper shows advantages and disadvantages each of those methods. The best approach to monitoring MRD in ALL during and after the treatment seems to be combination of molecular and immunological techniques.