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.