The ability of DNA to exist in configurations other than its classical double-stranded form has been known for many years. There has been a spectacular recent surge of interest in these forms, notably in the three-stranded or triple-helical form. Triplex-like nucleic acids are now known to exist in vivo, and may well participate in significant biological processes. Interest in triple-helical nucleic acids has been greatly stimulated by their potential exploitation to control gene expression, serve as tools in genome mapping strategies, etc. The authors have written an encyclopedic introduction to nucleic acid triplexes based on many years of familiarity with the topic. The book includes information on chemistry, conformation, physical properties, applications, and hypotheses about the biological role of triplexes. It pays particular attention to the different methods for investigating these molecules, a feature which will be welcomed by those new to the field.