Magnetism, Volume I: Magnetic Ions in Insulators: Their Interactions, Resonances, and Optical Properties summarizes the understanding of magnetically ordered materials. This book contains 12 chapters that specifically tackle the concepts of ferromagnetism, ferrimagnetism, and antiferromagnetism. After briefly dealing with the spin Hamiltonians of typical ions and the interactions between the ions, this book goes on discussing the diverse aspects of ferromagnetism, ferrimagnetism, and antiferromagnetism in insulators as well as in metals. These topics are followed by presentation of abstract quantum mechanical and statistical models and the theory of spin interactions in solids. The other chapters describe the actual magnetic structures and the phenomenology of ferromagnets. This text further considers the fundamentals of neutron diffraction and optical phenomena in magnetically ordered materials. The concluding chapters look into the cooperative phenomena characterized by ordered arrangements of magnetic moments subject to strong mutual interactions. Physicists and magnetism researchers will find this book of great value.