Recent advances in immunology and molecular biology have made considerable contribution to the diagnosis of lymphoma, which is a neoplasm of the immune system. Molecular diagnosis can be defined as "diagnostic testing directed at markers consisting of nucleotide sequences within DNA and RNA" (See reference 9.). The examination is now routinely applied to lymphoma diagnosis to identify the clonality, to estimate the cell of origin and normal counterpart, to investigate the etiology and pathogenesis including tumor progression, to detect minimal residual disease, and to evaluate the patient's prognosis. By this testing, oncologists and pathologists can obtain much important information for more objective diagnosis and appropriate management of patients with lymphomas. Although conventional examinations, such as histology, immunohistochemistry, and cytogenetics, are still required for selection of molecular genetic examinations and accurate interpretation of the results, they are undoubtedly a powerful tool for lymphoma diagnostics.