The use of in situ hybridization for light-microscopic demonstration of specific DNA or RNA-sequences is illustrated with examples. This method is useful for demonstration of viruses and gene products (mRNA) in individual cells in tissue sections or in cells in suspension. In situ hybridization technology is particularly useful in diagnostic pathology as an adjunct to the conventional methods for infectious diseases. In addition, the method is a powerful tool for analysing the interaction between viral infection and the induction and maintenance of certain human neoplasms. We discuss various aspects of tissue handling, fixation, hybridization procedures and detection of the hybridization signal. We also stress the importance of close cooperation between laboratories in the field of gene technology.