The polymerase chain reaction (PCR) allows the in vitro amplification of DNA fragments starting with tiny amounts of biological sample and oligonucleotide primers derived from sequence data. Since the technique is fast and easy, PCR has taken the DNA-technology to the routine laboratoria. We present a survey of the following applications of PCR: 1) The amplification of gene fragments as fast alternative of cloning. 2) The modification of DNA fragments. 3) The sensitive detection of pathogenic microorganisms, if desired followed by an accurate genotyping. 4) DNA analysis of arachaeological specimens. 5) The detection of mutations relevant for inherited diseases, malignant transformation or tissue typing. 6) The analysis of genetic markers for forensic applications, for paternity testing and for the mapping of hereditary traits. 7) The species-specific amplification of DNA segments between interspersed-repeat elements. 8) The study of gene expression.