The topics covered provide a qualitative description of the wide-band dielectric spectra of polar liquids and gases, molecular libration-rotation (which is the reason for dielectric loss and absorption of electromagnetic waves), simple molecular models and a comparison between the theoretical and experimental dependencies and derivation of the main results. A new feature is the application of a number of simple analytic models to different classes of substances including strongly absorbing liquids, solutions of polar molecules in nonpolar solvents, liquid water, water bound by macromolecules and aqueous solutions.