The development of a new technology can lead to novel approaches to answering current scientific questions. A new technology can also often lead to the framing of scientific questions that previously were considered intractable and not amenable to experimental investigation. The recombinant DNA revolution, Southern blotting and DNA sequencing are examples of technologies that have had such an impact on the biological sciences. Recently, a new molecular biological technique, the polymerase chain reaction (PCR) was developed. This method of in vitro gene amplification can in many circumstances simplify the standard procedures for cloning, analyzing and modifying nucleic acids. However, besides simplifying many recombinant DNA methodologies, PCR has opened the door to the investigation of a number of biological questions that heretofore were considered unanswerable. In this review, the basic principles of PCR along with modifications of the basic scheme will be discussed. The application of these methods to a number of standard genetic engineering procedures will be reviewed. Important factors that need to be taken into consideration in designing experiments using PCR are also examined.