Principles of the polymerase chain reaction (PCR) and its modifications are discussed: reverse transcription-PCR (RT-PCR), nested PCR, restriction fragment length polymorphism-PCR (RFLP-PCR). Different methods of the PCR products detection used in routine diagnostics and research are presented: electrophoresis and chromatography techniques, immunoenzymatic methods, hybridisation and sequencing. Applications of PCR reaction in diagnostics of infectious diseases (viral and bacterial), genetic diseases and genotyping in transplantation are discussed and examples of commercial tests based on PCR reaction and another molecular biology methods used in fast, routine diagnostics are listed.