This book provides a comprehensive account of fiber-optic communication systems. The 3rd edition of this book is used worldwide as a textbook in many universities. This 4thedition incorporates recent advances that have occurred, in particular, two new chapters. One deals with the advanced modulation formats (such as DPSK, QPSK, and QAM) that are increasingly being used for improving the spectral efficiency of WDM lightwave systems. The second chapter focuses on new techniques such as all-optical regeneration that is under development and likely to be used in future communication systems. All other chapters are updated, as well.