Teaching a course on nucleic acid structure is a hazardous undertaking, especially if one has no continuous teaching obligations. I still have done it on several occasions in various French universities, when colleagues, suffering from admin istrative overwork and excessive teaching obligations, had asked me to do so. This was generally done with a pile of notes and a dozen slides, and I always regretted that no small, concise, specialized book on nucleic acid structure for students at the senior or beginning graduate level ex isted. Every year, the lecture notes became more and more voluminous, with some key reprints intermingled. Everything changed when, in the spring of 1973, I re ceived an invitation to teach such a course, under the UNESCO-OAS-Molecular Biology Program at the Universi dad de Chile in Santiago during October 1973. I had ac cepted rather enthusiastically, but soon discovered that it would be necessary to produce a photocopied syllabus for the students. This was the fi rst premanuscript of this book. For nonscientific reasons, the course was first canceled and then postponed until December 1973. Nearly a year later, the course, in slightly amended form, was presented at the Lemonossow-State University in Moscow.