The Polymerase Chain Reaction is a technique in molecular biology, that allows million-fold amplification of DNA-fragments. It is based on specific oligonucleotides used as starting segments (primers) of both sides of the genomic region to be studied. This chain reaction is characterized by three steps: denaturation of DNA by increasing the temperature; renaturation of the DNA allowing for competition between the two original DNA segments and the numerous primers, and finally synthesis of DNA by the polymerase. The cycle is repeated from 25 to 35 times in order to exponentially duplicate the DNA-fragments between the two primers. Usage of a thermoresistant polymerase permitted automatization of the procedure. This technique represents a revolution for practical molecular biology and medicine. Its very high sensitivity, however, may cause errors that have to be recognized. Several practical examples are described and analyzed in order to illustrate the PCR-concept to physicians in practise.