First printed in 1934, this book gives a critical survey of the principles and limitations of methods for studying resonance radiation that is the re-emission of absorbed radiation without change of wavelength and of the theoretical interpretation of the phenomenon. At the time of first publication, physicists had come to the limit of the then existing techniques in the field and the book was more useful to astronomers who were interested in line shapes existing in the spectra of stars. In past years other fields and techniques have developed for which the information in this book is of interest. These fields are: (1) those having to do with the determination of spins, magnetic moments and hyperfine structure separations by the methods of 'optical pumping' and 'optical double resonance': (2) optical MASERS; and (3) nuclear resonance absorption of gamma rays.