Exploring the theory of polarization propagators for studying molecular structures and properties, this remarkable book connects the three common frameworks for study: non-relativistic, relativistic, and quantum electrodynamics. Combining classical mechanics with the observed wave nature of quantum particles, chapters range from basic theory to advanced subjects, computational methods, and experimental aspects. It bridges the gap between theoretical and practical aspects and includes in-text problems with solutions and case studies as well as online access to additional pedagogical material, exercises, and elementary Fortran code.