Liquid biopsy has become an opportunity in lymphoma diagnostics, since plasma circulating tumor DNA (ctDNA) is an easily accessible source of tumor DNA, which can be complementary to tissue biopsy. Through ctDNA, lymphomas can be molecularly characterized, tumor clonal evolution can be tracked, while monitoring minimal residual diseases during and after therapy. Here, we describe the methodology of cancer personalized profiling by deep sequencing (CAPP-seq) that we use for ctDNA qualification and quantification.