Book excerpt: The invention of the semiconductor laser along with silica glass fiber has enabled an incredible revolution in global communication infrastructure of direct benefit to all. Development of devices and system concepts that exploit the same fundamental light-matter interaction continues. Researchers and technologists are pursuing a broad range of emerging applications, everything from automobile collision avoidance to secure quantum key distribution. This book sets out to summarize key aspects of semiconductor laser device physics and principles of laser operation. It provides a convenient reference and essential knowledge to be understood before exploring more sophisticated device concepts. The contents serve as a foundation for scientists and engineers, without the need to invest in specialized detailed study. Supplementary material in the form of MATLAB is available for numerically generated figures.