The book is intended for graduate students and researchers who wish to master the main properties of magnetic materials in the bulk state and at the nanometric scale such as for thin films and multilayers. This textbook provides the theories and methods of simulation to study and to understand these properties in an explicit manner. In the first part of the book, the quantum theory of magnetism is presented while the second part of the book is devoted to the application of the theory of magnetism to surface physics. Numerous examples covering typical cases in ferromagnets, antiferromagnets, ferrimagnets, helimagnets, and frustrated spin systems are all illustrated. Fundamental surface effects are shown and discussed. Lastly, the spin transport is described - in which the basic formulation of the Boltzmann's equation is recalled - and the recent methods of Monte Carlo simulation to deal with the spin resistivity are explained.This book contains a large number of detailed solutions for the problems given in each chapter to help readers discover new related phenomena and applications, as well as an appendix on elements of statistical physics included at the end to make the book self-contained.