The majority of human infections in the United States are now caused by viruses. Viral infections range in severity from invariably fatal (rabies) to subclinical (echovirus). Inasmuch as one virus can cause a variety of syndromes, determination of the causative agent is essential for definitive diagnosis. During the last two decades, viral diagnostic procedures, especially the cell culture technic, have been greatly improved and simplified. Moreover, a microtiter system requiring much smaller amounts of reagents has been introduced. Thus, it is now financially and technically possible for a hospital with well-equipped microbiological laboratories to engage in diagnostic virology. Viruses have been classified into 13 groups on the basis of physical and chemical properties. Certain general features of the pathogenesis and pathology of viral diseases are presented; a good understanding of these aspects is necessary for the effective operation of a diagnostic virology laboratory. Four major viral diseases of the fetus and neonate, namely, rubella, cytomegalovirus, herpes simplex and enterovirus infections, which are of current interest, are briefly discussed.