The topics covered in this book provide a qualitative and sometimes quantitative classic description of the wide-band 0-THz dielectric spectra of polar liquids, molecular libration-rotation (which is the reason for dielectric loss and absorption of electromagnetic waves), simple molecular models differing by the intermolecular-potential profiles, and present a comparison between the theoretical and experimental dependencies and derivation of the main results. A new feature is the application of a number of analytical models to different substances, including strongly absorbing nonassociating liquids, liquid water, water bound by macromolecules, and gas-like liquids. The presentation of the theory in this book is also new. It is based on the dynamic method in which the Brownian reorientations are considered implicitly, without direct solution of stochastic equations. This approach simplifies the theory. Senior students and experimentalists will find many of the results valuable.