Polymerase chain reaction (PCR) is an efficient and one of the most common methods used in biological sciences for in vitro multiplication of a target DNA molecule. The technique has significantly contributed in changing and developing different fields of biological sciences since 1980s. PCR has a vital role in supporting the processes involved in genetic engineering, particularly the cloning of DNA fragments used to modify the genomes of microorganisms, animals, and plants. Consequently, the technique has numerous applications in fundamental and applied research in medicine agriculture, environment, and bio-industry. The main focus of this chapter is to describe briefly the principles, methodology, various types, and applications of PCR in different fields. Besides, different components of PCR, trouble shooting during the execution, and limitations of the techniques are also outlined.