Book excerpt: Devices, nanoscale science and technologies based on GaN and related materials, have achieved great developments in recent years. New GaN-based devices such as UV detectors, fast p-HEMT and microwave devices are developed far more superior than other semiconductor materials-based devices.Written by renowned experts, the review chapters in this book cover the most important topics and achievements in recent years, discuss progress made by different groups, and suggest future directions. Each chapter also describes the basis of theory and experiment.This book is an invaluable resource for device design and processing engineers, material growers and evaluators, postgraduates and scientists as well as newcomers in the GaN field.