PCR is a patented method that allows specific logarithmic amplification of DNA. Smallest traces, even single moelcules can be specifically identified and simplified for further use. This text provides descriptions of application and corresponding protocols of the polymerase chain reaction. The applications cover a wide range from diagnosis and therapy in clinical medicine over standard techniques used in molecular biology research to highly specialized applications in the fields like archaeology or forensic medicine. A wide range of biological systems and species is described, from micro-organisms to humans.