Applications of both short-and long-period fiber gratings are very diverse, especially the ones related to the optical communications and the optical fiber sensing. In optical communications different tasks such as wavelength selective filtering, add/drop multiplexing and all-optical switching have been performed. Interestingly, optical fiber sensing has unique advantages such as immunity to electromagnetic radiation, high sensitivity, cost-effective relation, etc. And it is widely used to measure stress, temperature, pressure or chemical parameters, among others physical variables, giving in many cases unrivaled resolution capabilities. In this book the reader will find a collection of chapters written by different experts around the world, covering the broad field of fiber gratings. The main goal of this book is to give an updated overview of the current research trends in the rapidly evolving field of fiber gratings.