The polymerase chain reaction (PCR) is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence. PCR is now a common and often indispensable technique used in medical and biological research labs for a variety of applications. There are three major steps involved in the PCR technique: denaturation, annealing, and extension. PCR is useful in the investigation and diagnosis of a growing number of diseases. Qualitative PCR can be used to detect not only human genes but also genes of bacteria and viruses. PCR is also used in forensics laboratories and is especially useful because only a tiny amount of original DNA is required. PCR can identify genes that have been implicated in the development of cancer. Molecular cloning has benefited from the emergence of PCR as a technique. The present paper is an attempt to review basics of PCR.