1. The Discovery of Triple-Stranded Nucleic Acids.- Basic Physicochemical Properties of Nucleic Acids.- Bases, Nucleosides, Nucleotides, Polynucleotides.- Base Pairs and Double-Stranded Nucleic Acids.- Some Methods of Investigating Nucleic Acids.- Circular and Superhelical DNA.- Physicochemical Studies of Model Triple-Stranded Structures.- The Discovery of Nucleic Acid Triplexes.- Physicochemical Studies of Model Triplexes.- Early Hypotheses About the Biological Roles of Triple-Stranded Nucleic Acids.- Studies of Nuclease S1 Susceptibility of Specific DNA Sequences.- Experimental Evidence That the S1-Sensitive PyPu Tracts in Supercoiled DNA Form Intermolecular Triplexes.- Intermolecular Triplexes Between DNA and Oligonucleotides.- Current Fields of Interest in Investigation of Triplexes.- 2. Methods of Triplex Study.- Physical Methods for Triplex Study.- Spectral Methods.- Differential Scanning Calorimetry.- Equilibrium Sedimentation.- Electrophoretic Techniques.- Immunological Methods.- Affinity Methods.- Affinity Chromatography.- Filter-Binding Assay.- Electron Microscopy.- X-Ray Analysis.- A Short Overview of Experimental Physical Methods for Triplex Studies.- Theoretical Descriptions of the Triplex Structures.- Enzymatic and Chemical Probing of Triplex Structures.- Analysis of Modifications Using Maxam—Gilbert Sequencing Gel Analysis.- Primer Extension Analysis.- Enzymatic Methods.- Single-Strand-Specific Nucleases.- DNase I Footprinting.- Inhibition of Restriction Nuclease Action.- Chemical Methods.- Unbound Agents.- Photofootprinting.- A Short Overview of Chemical and Enzymatic Probes.- Site-Directed Agents.- Nuclease-Like Oligonucleotides.- Photoactive Groups Attached to Oligonucleotides.- A Short Overview of Site-Directed Agents.- Conclusion.- 3. General Features of Triplex Structures.- Basic Types of Triplexes.- Nucleotide Sequence Requirements.- Intermolecular and Intramolecular Triplexes.- Molecular Details of Triplex Structure.- Major-Groove Location of the Third Strand.- Base Triads in Nucleic Acids.- Orientation of the Third Strand.- Overall Conformations of Triplexes.- Stabilization Common to Intramolecular and Intermolecular Triplexes.- Factors That Destabilize Triplexes.- Specific Features of Intramolecular Triplexes (H and H* Forms).- Specific Features of Intermolecular Triplexes.- DNA—RNA Triplexes.- Kinetics of Triplex Formation.- Thermodynamics of Triplexes.- New Variants of Triplex Structure.- Conclusion.- 4. Triplex Recognition.- Extension of Triplex Recognition Schemes.- Natural Bases in Unusual Triads.- Triplexes with Base and Nucleoside Analogs in the Third Strand.- Abasic Sites in the Third Strands.- Base Analog in the Duplex Part of the Triplex.- Alternate Strand Triplex Formation.- Modified Oligomer Backbones.- Modifications in the Sugar—Phosphate Backbone.- Conjugated Oligonucleotides.- Protein—DNA Interactions and Triplex Formation.- Triplex DNA—Drug Interactions.- Groove—Binders.- Intercalators.- Conclusion.- 5. The Forces Participating in Triplex Stabilization.- Triplex Stabilizing Factors.- Reduction of Interstrand Repulsion.- pH Stabilization.- Length-Dependence.- Differential Effect of Divalent Cations.- The Hydration State of Nucleic Acids.- Hydrophobic Substituents in the Third Strand.- Possible Interactions Which Favor and Stabilize Triplexes.- Electrostatic Forces.- Stacking Interactions.- Hoogsteen Hydrogen Bonds.- Hoogsteen Hydrogen Bond Enhancement.- Hydration Forces.- Contribution of Hydrophobicity.- Interrelation of Different Triplex-Stabilizing Contributions.- Conclusion.- 6. In Vivo Significance of Triple-Stranded Nucleic Acid Structures.- In Vivo Existence of Triplexes.- Search for Triplexes in the Cell.- Factors That Could Be Responsible for Triplex Formation In Vivo.- Possible Biological Roles of Triplexes.- Possible Regulation of Transcription.- Possible Regulation of Replication.- Possible Triplex-Mediated Chromosome Folding.- Structural Role at Chromosome Ends.- Recombination.- Possible Role in Mutational Processes.- Do PyPu Tracts Play a Role in RNA Splicing?.- Elements of Triple-Stranded Structure in RNA.- Other Roles of the PyPu Tracts.- Coding of Charged or Hydrophobic Amino Acid Clusters.- Can the PyPu Tracts Exclude Nucleosomes from Certain Gene Regions?.- Conclusion.- 7. Possible Spheres of Application of Intermolecular Triplexes.- Applications of Intermolecular Triplex Methodology.- Extraction and Purification of the Specific Nucleotide Sequences.- Affinity Chromatography.- Quantitation of Polymerase Chain Reaction Products.- Nonenzymatic Ligation of Double-Helical DNA Mediated by Triple Helix Formation.- Triplex-Mediated Inhibition of Viral DNA Integration.- Site-Directed Mutagenesis.- Detection of Mutations in Homopurine DNA Sequences.- Mapping of Genomic DNA.- Control of Gene Expression.- Conclusion.- References.