Group VII of the Landolt-Börnstein New series is devoted to the physical properties of biological systems. It begins with volume VII/1 covering the nucleic acids which are of central importance in all processes involving gene expression. This field has become of practical consequence during the last decade through the development of genetic engineering. Because of the amount of the data available, volume VII/1 had to be divided into several subvolumes. The first two subvolumes, VII/1a and VII/1b, deal with crystallographic and structural data which give a comprehensive information on the three-dimensional structure of the nucleic acids, their folding properties, their hydrogen bonding schemes, their metal binding capabilities, their hydration and their drug complexation. The present subvolume VII/1a treats nomenclature and definitions in the field of nucleic acids, followed by the structure data of bases, nucleosides, nucleotides, and oligonucleotides. The basic foundations of nucleic acid structure are laid in this subvolume. Subvolume VII/1b which is to follow shortly, is concerned with the relevant data on fiber structures of DNA and RNA, on complexes between nucleotides and metal-ions and between oligonucleotides and drugs, and on hydrogen bonding of the nucleic acid constituents.