Polymerase Chain Reaction (PCR) is a new, popular, molecular, biological technique for enzymatic replicating DNA without using a living organism. PCR is commonly used in medical and biomedical researches for diagnosis of different infectious diseases like tuberculosis, HIV, hepatitis, fungal and protozoal diseases. Also, PCR is used for identification of genetic fingerprints and the crime suspects in criminal offences. This chapter deals with indications, basic requirements, procedure and recent modification in PCR techniques like nested PCR, inverse PCR, RT PCR, asymmetric PCR and quantitative PCR.