This book describes in a comprehensive manner the components and systems of non-linear fiber optic communications system. The first section explains the theory of single-mode fibers, then the nonlinear issues, including cross phase modulation, self-phase modulation, four wave maxing, stimulated briliouin scattering and stimulated raman scattering. The second section describes the Power budget; power penalty and cost benefit analysis for long-haul high capacity optical communication networks. Finally, the optical transmission system design is explained, and techniques how to mitigate non-linear issues, and increasing the system capacity in terms of data rate speed, path cover and bandwidth capacity.