Semiconductor laser diodes are the smallest, most reliable, and most efficient light source known to man. For that reason, they are commercially interesting devices. Laser diodes have created multibillion dollar industries in optical communications and optical information storage. They are helping to create new industries in optical trace-gas sensing, metrology, biological assays (including DNA analysis) and machining applications such as soldering, welding and cutting. The applications will continue increase as researchers and manufacturers improve performance and reduce costs. These developments will depend, in a large part, on demands from users who understand how these devices work and have a realistic picture of how to improve them for specific applications.