The polymerase chain reaction (PCR) is a technique that enables us to prepare in a test tube billions of copies of a specific piece of DNA. The principles and practical aspects of the PCR are exposed. Within veterinary medicine the most important applications are expected to be i) the diagnostics of infectious diseases and ii) the detection of genetic disorders or desired genetic traits. The PCR can be carried out on a multitude of samples ranging from mummies to paraffine-embedded tissue specimens. However, inhibitors present in e.g. serum, urine and faeces have to be removed.