Despite several developed treatment protocols used for treatment of patient with precursor B-cell leukemia/lymphoma, many of these patients relapse. Most of these patients relapse from clinically inapparent minimal residual disease (MRD), which refers to the presence of clonal cells in BM after treatment in a level below the morphological detection. The ability to predict who will relapse while still in a minimal disease state has important clinical implications. A reliable technique for MRD detection should be specific, sensitive, reproducible and quantitative. Currently, PCR analysis of Ag receptor gene rearrangements and flowcytometry represent the most clinically useful targets for MRD analysis in ALL. Molecular techniques may be sequence- or non sequence based methods. This book will discuss the basic knowledge about precursor B-cell neoplasms, MRD and different molecular methods for its detection with stress on a non-sequence based technique. This book should be useful for both medical student and medical professional in the field of laboratory or clinical hematology.