The semiconductor laser‎,‎ invented over 50 years ago‎,‎ has had an enormous impact on the digital technologies that now dominate so many applications in business‎,‎ commerce and the home‎.‎ The laser is used in all types of optical fibre communication networks that enable the operation of the internet‎,‎ e‎-mail‎,‎ voice and skype transmission‎.‎ Approximately one billion are produced each year for a market valued at around $5 billion‎.‎ Nearly all semiconductor lasers now use extremely thin layers of light emitting materials ‎(‎quantum well lasers‎)‎‎.‎ Increasingly smaller nanostructures are used in the form of quantum dots‎.‎ The impact of the semiconductor laser is surprising in the light of the complexity of the physical processes that determine the operation of every device‎.‎ This text takes the reader from the fundamental optical gain