The assessment of the presence of clonal lymphoproliferations via polymerase chain reaction (PCR)-based analysis of rearranged immunoglobulin (Ig) or T-cell receptor (TCR) genes is a valuable technique in the diagnosis of suspect lymphoproliferative disorders. Furthermore this technique is more and more used to evaluate dissemination of non-Hodgkin lymphoma and/or the presence of (minimal) residual disease. In this chapter we describe an integrated approach to assess clonality via analysis of Ig heavy chain (IGH), Ig kappa (IGK), TCR beta (TCRB), and TCR gamma (TCRG) gene rearrangements. The described PCR protocol is based on the standardized multiplex PCRs as developed by the European BIOMED-2 collaborative study (Concerted Action BMH4-CT98-3936). Furthermore it also includes the pre-analytical DNA isolation step from various tissues (formalin fixed paraffin-embedded tissue, fresh tissues, body fluids, peripheral blood and bone marrow), GeneScan analysis of labeled PCR products on a genetic analyzer, heteroduplex analysis of unlabeled PCR products, and post-analytical guidelines for the interpretation of the obtained “molecular morphology” patterns.