Nucleic acid hybridization is a recently developed laboratory technique that allows identification of the genetic material in tissue specimens. The role of the nucleic acids DNA and RNA in cellular function and disease is reviewed, followed by discussion of nucleic acid detection techniques. Earlier methods used to detect nucleic acids were slow and time-consuming. Current in situ detection techniques allow rapid characterization of pathogenic organisms in tissue sections and localization of the pathologic genetic material to specific cellular regions. This technology has been useful in the understanding of viral ocular diseases including herpes keratitis and cytomegalovirus retinitis and also has great potential in the understanding of the pathogenesis of human cancers, genetic disorders and endocrine and immunologic diseases.