This book summarises recent progress in the science and technology of rare-earth doped nitrides, providing a snapshot of the field at a critical point in its development. Improved theoretical understanding of the behaviour of f-electrons in solids, driven by accelerating computational capability, has coincided with an improved ability of crystal growers to incorporate rare earth elements reproducibly in wide bandgap semiconductor hosts that are favoured for optoelectronic and spintronic applications. The device possibilities of these advanced materials are promising but by no means well-developed. Following a review of the theoretical background, this book explores the preparation of materials, either through in-situ growth or ion implantation/annealing, the characterisation of these materials and their incorporation in existing and prospective devices. Featuring contributions from leading researchers in Europe, the USA and Japan, the book will be a valuable resource for academics, advanced students and industrial scientists.