Minimal residual disease refers to the postremission persistence of leukemia that cannot be detected by histomorphologic assessment. Techniques for MRD detection can be classified based on the type of cell marker used to identify the malignant cell clone, as follows: Morphology, cell culture techniques that evaluate the pattern of in vitro colony growth, cytogenetic characteristic of blast cells evaluated either by conventional chromosome analysis or by fluorescence in situ hybridization (FISH), the immunophenotypic profile of blast cells assessed by multiparametric flow cytometry, DNA sequences analyzed by polymerase chain reaction (PCR), Gene expression pattern of genes relevant to tumor development assessed by microarray methodology. The efficacy and applicability of the different methodological approaches that are available for the detection of MRD depend on three main features : -Specificity: discrimination between malignant and normal cells, without false-negative and false-positive results.-Sensitivity: the detection limit of a MRD technique must be at least 10ˉ³ i.e.discrimination of 1 leukemic cell among 1000 normal cells. -Reproducibility: the technique should allow for easy standardization and rapid collection of results for their clinical application. Studies of minimal, or submicroscopic, residual disease (MRD) during treatment can accurately gauge treatment response and allow estimates of the residual leukemic cell burden during clinical remission in individual patient, thereby refining the selection of therapeutic strategies and, possibly, long-term clinical outcome.