Since the publication of Volumes II/7 in 1976 and its supplements II/15 in 1987 and II/21 in 1992, the information on the structure of free molecules in the ground state and in excited electronic states has increased considerably. Therefore this volume II/23 contains data from 148 inorganic and 498 organic polyatomic free molecules (including free radicals and molecular ions) published between 1990 and 1993 inclusively and a small number of structures published 1994. All experimental methods for the determination of structural data of free molecules have been considered, all data obtained by these methods have been critically evaluated and compiled. The structural data for more than 3400 polyatomic free molecules can be completely surveyed and easily retrieved by means of this volume.