PCR is a method used to prepare billions of copies of specific DNA sequences, i.e, to amplify a DNA sample. It is often necessary to have a larger number of copies of a specific DNA sequence than that found in a typical sample for further DNA analysis (for example, for DNA fingerprinting or genotyping).

The PCR reaction is highly specific, meaning that it will only produce copies of a desired sequence from the template (sample) DNA. This specificity is ensured by the primers, which are designed to be complementary and anneal to specific regions on each side of the DNA region of interest (target region).