This volume of Solid State Physics provides a broad review on recent advances in the field of magnetic insulators, ranging from new spin effects to thin film growth and high-frequency applications. It covers both theoretical and experimental progress. The topics include the use of magnetic insulators to produce and transfer spin currents, the excitation of spin waves in magnetic insulators by spin transfer torque, interplay between the spin and heat transports in magnetic insulator/normal metal heterostructures, nonlinear spin waves in thin films, development of high-quality nanometer thick films, and applications of magnetic insulators in rf, microwave, and terahertz devices, among others. The volume not only presents introductions and tutorials for those just entering the field, but also provides comprehensive yet timely summaries to specialists in the field. Solid-state physics is the branch of physics primarily devoted to the study of matter in its solid phase, especially at the atomic level. This prestigious series presents timely and state-of-the-art reviews pertaining to all aspects of solid-state physics.