Molecular diagnostics examines the basic molecules of the genome for relevant aberrations, according to diagnosis, prognosis and therapy. It depends mainly on PCR-based methods (qualitative and quantitative analysis, RT-PCR, nested PCR, melting curve analysis, sequencing etc.). The general practitioner or consultant in internal medicine encounters molecular diagnostics by his evaluations of the reasons for thromboembolism (mutation of factor V Leiden or prothrombin), myeloproliferative neoplasms (JAK2V617F-mutation, BCR-ABL1) or hemochromatosis. The specialist furthermore appreciates its value in diagnosis and treatment monitoring of acute and chronic leukemia, lymphoma, familial erythrocytosis and a variety of solid tumors (especially breast cancer and colorectal carcinoma), as well as in managing refractory transfusion answers, suspected disturbances in drug metabolism and in the planning of an allogeneic stem cell transplantation.