To the casual observer, in situ hybridization (ISH) suggests a method to detect and localize nucleic acid sequences in cell preparations, tissue sections or chromosome spreads. This paper is an attempt to demonstrate that ISH is very much more versatile than is immediately apparent; a number of possible alternatives in different steps in the procedure have been defined and specific methods recommended that are known to be successful. The text is limited to the detection of viral nucleic acids in tissue sections and cells, although many of the concepts may be applied more generally. Methods and practical details have been reduced to a minimum as they are covered in the accompanying paper by Dr. F. Negro et al. and in references. A comprehensive review of ISH procedures has been compiled recently.