The polymerase chain reaction is a recently introduced laboratory technique that allows for the identification of extremely small quantities of DNA. This technique offers significant advantages over more traditional methods of identifying infectious organisms causing ocular pathology. Increasing numbers of immunosuppressed and immunocompromised patients have led to the emergence of posterior segment infection from unusual and sometimes difficult to diagnose organisms. Improvements in antibiotic and antiviral therapy for specific organisms will require increasingly accurate diagnosis. Polymerase chain reaction is capable of providing this and is increasingly being used in the diagnosis of unusual vitreoretinal pathology.