Molecular methods for rapid microbiologic diagnosis, particularly polymerase chain reaction genomic amplification of microbial nucleic acids, have been developed for almost all known pathogenic bacteria, viruses, parasites, and fungi. Except for a few of them (human immunodeficiency virus, hepatitis B and C, cytomegalovirus, mycobacteria, and Chlamydia trachomatis), most of the other tests have not yet been moved from the research laboratory to routine clinical use. In particular, molecular tests designed to detect rapidly the causative agent(s) of life-threatening infections, such as septicemia and meningitis, have to be standardized and run in parallel with conventional tests to define their clinical utility and cost-effectiveness.