The polymerase chain reaction (PCR) is an in vitro method for the amplification of DNA that was introduced in 1985 (1). The principle of the PCR is elegantly simple but the resulting method is extremely powerful. The adoption of the thermostable Taq polymerase in 1988 greatly simplifies the process and enables the automatron of PCR (2). Since then a large number of applications have been developed that are based on the basic PCR theme. The versatility and speed of PCR have revolutionized molecular diagnostics, allowmg the realization of a number of applications that were impossible in the pre-PCR era. This chapter offers an mtroductory guide to the process.