This book covers the recent progress in fiber-optic communication systems with a main focus on the impact of fiber nonlinearities on the system performance. Over the past few years, there has been significant progress in coherent communication systems mainly because of the advances in digital signal processing techniques. This has led to renewed interest in fiber linear and nonlinear impairments and techniques to mitigate them in electrical domain. In this book, the reader will find all the important topics of fiber optic communication systems in one place with in-depth coverage by the experts of each subtopics. Pioneers from each of the sub-topics have been invited to contribute. Each chapter will have a section on fundamentals, review of literature survey and the recent developments. The reader will benefit from this approach since many of the conference proceedings and journal articles mainly focus on the authors' research work without spending space on preliminaries.