This book was written from lectures given to MSc students following the 'Matter and Radiation' course at the University of Grenoble I. Although magnetism and superconductivity cover a wide area of physics, the course was motivated by a common factor: these phenomena are realisa tions of thermodynamic states which break certain continuous symmetries. In the case of magnetism, they break rotational invariance. In the case of superconductivity, they break gauge invariance. The aim of the course was to bring out the importance of broken symmetries in condensed matter physics. The book can be understood with minimal prerequisites and the math ematical techniques used are fairly elementary. However, a basic knowledge of spin and angular momentum is essential, since quantum mechanics lies at the heart of both magnetism and superconductivity. Chapter 2 reviews the main points. The first chapter explains how thermodynamic functions are constructed in the presence of a magnetic field. As the book has two parts, Magnetism (I) and Superconductivity (II), these will be specified between brackets in cross-references to sections and chapters. I have made a particular effort to present phenomena in magnetism and superconductivity by starting with concrete examples. Some technological applications of superconductivity have also been described."