Optical amplifiers not only permit propagation of lightwave signals over thousand of kilometers, they also enables the use of massive WDM system and have led to the development of lightwave systems with capacities exceeding 1Tb/s. Fiber nonlinearities plays a very important role in designing such high capacity lightwave system. Two such nonlinear effects, cross-phase modulation (XPM) and self-phase modulation (SPM) are the prime subjects of concern in this book. XPM and SPM are two phase modulation phenomena subjected to the nonlinear intensity dependence of the core refractive index. Dispersion causes these phase modulations to appear as intensity variation of the propagating signal at the receiving end of the system. This book is based on the extensive analytical study on Cross-Phase Modulation (XPM) and Self-Phase Modulation (SPM), which have a very significant effect on today's high speed DWDM system. Throughout this book, the authors have presented their research outcome and relevant theoretical overview. This book will help, researchers and students who are working with the performance analysis of optical system due to different fiber nonlinearities.