Methods of Experimental Physics, Volume 3, Part A: Molecular Physics, Second Edition discusses the origin of the molecular theory and its application to the determination of molecular structure. This three-chapter book addresses the molecular lasing system and the dynamics of active plasma. Some of the topics covered in the book are the description of electronic states; general theory of infrared spectra; velocity of light determination; tunable devices for the generation of coherent submillimeter power; and analysis of electronic transition. Other chapters deal with the measurements of sound speeds and relaxation frequency by means of light scattering. These topics are followed by discussions of the historical introduction to spontaneous Raman scattering and the analysis of the Rayleigh Wing scattering. The last chapters describe the nuclear quadrupole hyperfine structure and the submillimeter stimulated emission devices. The book can provide useful information to chemists, physicists, students, and researchers.