Book excerpt: Sequence-specific DNA binding ligands, amongst which triple helix forming oligonucleotides are the most efficient as yet, represent promising tools in a number of fields. One of their most promising applications is as antiviral tools: they can specifically target a viral gene, even if it is integrated into the host genome, and be used to specifically inactivate the viral gene or even destroy the cells harboring this gene. However, from science fiction to science there remains a gap; and we are at the moment on the threshold of this fascinating field. Triple Helix Forming Oligonucleotides considers the different aspects of the design and improvement, current or future, of these molecules and their structural analysis, as well as their applications, with special emphasis on the attempts to obtain biological effects of these potentially important tools. What emerges is that the current state of the research is encouraging, and that these molecules are already useful in some biotechnology applications.