This book describes the key aspects of semiconductor laser device physics and principles of laser operation. It is an accessible, convenient reference and essential knowledge that may be easily understood before exploring more sophisticated device concepts. The contents serve as an essential foundation for scientists and engineers about how semiconductor lasers work and the fundamentals determining their behavior without requiring highly specialized and detailed study. New material in the second edition includes expanded and improved descriptions of basic concepts and practical applications. A chapter on quantum effects in small semiconductor lasers has been included, and appendices provide helpful supplemental material.