The microorganisms are the initiating factors of root canals infection. The relationship between the microorganisms in the infected root canals and those diseases cannot be ignored. Endodontic and periradular infections can be diagnosed as endodontitis, apical periodontitis, periradicular abscesses, secondary/persistent endodontic infections etc.. As sensitive and specific polymerase chain reaction（PCR） technique used in the field of microorganisms in the root canal, microorganisms in different forms of infected root canals are investigated by PCR, which are summarized in this review.
