In situ hybridization (ISH) combines molecular biological techniques with histological and cytological analysis of gene expression. RNA and DNA can be readily localized in specific cells with this method. ISH has been useful as a research tool, and recent studies have used this technique in the diagnostic pathology laboratory and in microbiology for the tissue localization in infectious agents. Other recent developments in the applications of ISH involve in situ polymerase chain reaction (PCR) and in situ reverse transcription (RT)-PCR, which can be used to detect very low levels of nucleic acids in tissues by taking advantage of the powerful amplification capacity of PCR. In situ PCR will contribute significantly to progress in this field because of the marked increase in the sensitivity of this method.