The present book contains essentially the material of a lecture series on the theory of magnetism, which was held for students of physics at a medium advanced level. The set-up follows the natural subdivision of magnetic substances, i.e. diamagnetism was treated first, then paramagnetism, and finally ee dealt with ferromagnetism, antiferromagnetism and ferrimegnetism. Naturally some parts of this field, magnetism, had to be dealt with in less detail than other parts, as the volume of the book was limited. Thus in the first part of the book we have put emphasis on the de Hassv-van Alpen effect, which iw very instructive as to the behavior of electrons in crystals. In the second part the influence of the crystal field on tha paramagnetism of atoms and connected problems (Kramers theorem, adiabatic demagnetization, paramagnetism resonance, ...) have been dealt with more extensively.