Forensic science and DNA typing laboratories have greatly benefited from the discovery of a technique known as the polymerase chain reaction (PCR). The PCR DNA amplification technology is well suited to the analysis of forensic DNA samples because it is sensitive, rapid, and not limited by the quality of the DNA as are the restriction fragment length polymorphism methods. PCR is an enzymatic process in which a specific region of DNA is replicated over and over again to yield many copies of a particular sequence. PCR is simplified in recent years by the availability of reagent kits that allow a forensic DNA laboratory to add a DNA template to a premade PCR mix containing all the necessary components for the amplification reaction. The most important components of a PCR reaction are two primers that are short DNA sequences that precede or flank the region to be copied. Controls are used to monitor the effectiveness of the chosen experimental conditions and/or the technique of the experimenter. These controls typically include a negative control and positive control. When amplifying very low levels of DNA template, a phenomenon known as stochastic fluctuation can occur. The instrument that heats and cools a DNA sample in order to perform the PCR reaction is known as a thermal cycler. Well-designed primers are probably the most important components of a good PCR reaction. This chapter discusses many available designs and explains multiple PCRs. Contamination of PCR reactions is always a concern because the technique is very sensitive to low amounts of DNA. Some tips for avoiding contamination with PCR reactions in a laboratory setting are given in the chapter.