Even in optical communications, lasers are used in different ways, from metropolitan links using directly modulated devices to 100 Gb/s transmission systems incorporating advanced detection and modulation schemes. In this book, I introduce semiconductor lasers from an operational perspective to those who have a background in engineering or optics, but no familiarity with lasers. The objective here is to present semiconductor lasers in a way that is both accessible and interesting to advanced undergraduate students and to first-year graduate students. The target audience for this book is someone who is potentially interested in careers in semiconductor lasers, and the decision of what topic to cover is driven both by the importance of the topic and how fundamental it is to the whole field.