The polymerase chain reaction (PCR) uses in vitro enzymatic synthesis to amplify specific DNA sequences. PCR amplification can produce approximately 100 billion copies of one molecule of DNA in a few hours. PCR has revolutionized research in the biological sciences and medicine, and has influenced criminology and law. Several major scientific discoveries, including purification of DNA polymerase and elucidation of the mechanism of DNA replication, were essential for development of the present PCR technology. An overview of these discoveries and early work on in vitro DNA synthesis are presented. Basic PCR methodology, instrumentation, advanced PCR techniques, and applications are also discussed in this review. Several new amplification systems are mentioned. PCR is an extremely important and simple technology for research and diagnostic analyses of DNA and RNA. PCR technology and other amplification procedures will continue to produce novel applications in basic research and clinical medicine.