In this monograph we describe an important and relatively new class of phenomena in the field of high-resolution atomic spectroscopy: the interference effects manifest in the angular distribution and polarization of spontaneous radiation and absorption by atoms. Although the quantum-theoretical descrip­ tion of these interference effects is quite subtle, it turns out - as so often in quantum mechanics - that a simple classical or semi-classical description offers much insight and can even explain quantitative features. In this presentation, however, we attempt to give the full story. Beginning with the simple semi­ classical description, we then present the quantum-mechanical analysis based on the density-matrix formalism and the statistical tensor. The remaining two chapters discuss experimental observations and data analysis. A great variety of effects have now been observed and can be used to obtain highly accu­ rate information about hyperfine structure, atomic constants, interaction con­ stants, etc. The authors have assumed only a basic knowledge of quantum mechanics and electromagnetism, thus making the book accessible to those beginning a graduate studies program. It is also aimed at practising spectroscopists and all researchers for whom atomic spectroscopy is an important tool - for these readers it will hopefully offer some new solutions and ideas for furthering their research. February 1993 E. B. Alexandrov M. P. Chaika G. I. Khvostenko Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2. Classical Description of Interference Phenomena in Radiation 2. 1 The Classical Oscillator Model of Atomic Emission . .