1.Introduction. 2.The macroscopic approach. Derivation of Appleton’s equation by means of an equivalent continuous medium. 3.The microscopic approach. The physical reason for the characteristic waves and their dispersion. 4.Absorption on the ‘macroscopic theory’. 5.The microscopic picture of absorption. 6.Graphical representation when there are no collisions. 7.The polarizations of the waves. 8.The quasi-longitudinal (QL) and quasi-transverse (QL) approximations, 9.The effect of collisions on the curves for a and X. 10.Some representative curves for the refractive index and the absorption index (x). 11.The group velocity. 12.A model ionosphere with no magneto field. 13.A model ionosphere with a magnetic field. 14.The nature of the electrons motions in the ionosphere. 15.The Lorentz term. 16.Waves in a mixture of ions and electrons. 17.Inhomogeneous media. 18.A magneto-ionic medium considered as an anisotropic dielectric, and a comparison with crystal optics.