In the last few years, molecular hybridization methods have been used extensively in basic and applied virology because of their technical flexibility and high specificity. Using these techniques, the detection of DNA and RNA viruses directly from clinical specimens, the analysis of the specific transcriptional activity of viral genes in vitro and in vivo, and the study of virus-host relationships have all been carried out at the molecular level. However, although these methods are efficient for many purposes, only development (~) and optimization (z) of PCR amplification have dramatically improved sensitivity. Currently, PCR is the method of choice for the detection of viral nucleic acids present at very low amounts in biological samples, and it allows the molecular study of most acute and persistent viral infections. Impressive achievements that are rap