This textbook provides a comprehensive treatment of optical communication systems, one of the fundamental technologies in telecommunication networks, and the backbone of our communication and information society. Geared towards upper undergraduate and graduate students, this book explains key technologies and recent trends in long-haul optical communication systems. After a brief introduction, the authors present key technologies in optical fiber communication systems. They go on to discuss long-haul optical fiber transmission using regenerative repeaters or optical amplifier repeaters, dispersion managed optical fiber transmission considering fiber nonlinearity, high-capacity optical fiber transmission using wavelength division multiplexing, and high spectral efficiency transmission using digital coherent receivers, among others. The book includes homework problems and PowerPoint slides for easy integration into classroom use.