This chapter contains sections titled: Basic aspects of detection of minimal residual disease in acute leukaemia Minimal residual disease detection by immunology and flow cytometry Minimal residual disease detection by polymerase chain reaction PCR-based MRD detection using antigen receptor genes Clinical value of monitoring for antigen receptor gene rearrangements by PCR in ALL False-negative and false-positive MRD tests MRD detection in other lymphoid malignancies Chromosomal translocations as molecular targets for MRD analysis FLT-3 internal tandem repeats, c-KIT mutations and WT1 levels Conclusions