The genomes of the herpesviruses are large complex DNA molecules with several interesting and perhaps unique properties. In the past few years the structure of these genomes has generated considerable interest, and significant progress has been made in the elucidation of the structural organization of these DNA molecules. A particularly interesting feature is the presence within the molecules of tandem and inverted repeat sequences, as well as the presence in some of the herpesvirus genomes of components that differ in their orientations with respect to one another, giving rise to different isomers of the same molecule.