DURING the last two decades great progress has been made in the experimental study and theoretical interpretation of molecular spectra, which has thus become one of the most important means we have for investigating molecular structure. There is, indeed, no experimental method that gives more detailed information on this subject. From the spectra, the discrete energy states of the molecule, as well as the limits of energy continua, can be derived directly. From these in turn we can draw precise conclusions concerning the orbital motions of the electrons, and the vibrations and rotations of the nuclei. As to the electronic motion, we may learn its quantal type, including its symmetry and arrangement of nodes; we may deduce its energy as a function of the inter-nuclear distances, and its angular momentum; and we can evaluate any associated angular momentum due to an unbalanced relative orientation of electronic spins. This information has led to a deeper theoretical understanding of chemical bindings. As to nuclear vibrations, we can learn their frequencies, calculate the forces between the atoms, and derive also the energy required to bring about dissociation, as well as the dependence of the dipole moment and polarizability of the molecule on inter-nuclear separation. From rotational data we can derive very accurate values for the inter-nuclear distances, and in special cases obtain information concerning the spin and statistics of the nuclei. The detailed and quantitative knowledge of molecular constitution thus obtained leads, in its turn, to the possibility of calculating certain properties of matter in bulk; for example, in the fields of magneto-chemistry and thermodynamics; the total energy, free energy and specific heats of many gases can be derived more accurately from spectroscopic data than by direct thermal measurement. Other achievements of molecular spectroscopy in the chemical field include the detection and characterization of many molecules, radicals and ions, C2, He2, Na2, OH, CP, H+, HC1+ and some hundreds of others that were not previously thought capable of free existence, the elucidation of elementary chemical reactions, and the prediction of chemical equilibria. Molecular Spectra and Molecular Structure 1: Diatomic Molecules. By Prof. Gerhard Herzberg. Translated with the co-operation of the Author by Prof. J. W. T. Spinks. (Prentice-Hall Physics Series.) Pp. xxviii + 592. (New York: Prentice-Hall, Inc., 1939.) 6.50 dollars.