Laser operating principles are discussed in relation to specific laser systems and materials. The aim is to provide the student with the working knowledge to understand the most used laser systems and their tecnological evolution, as well as how to choose the most appropriate laser for a given application. In particular, the approach to solid-state laser design will be explained with some practical examples. Solid-state laser engineering involves today most of the professionals designing laser sources or optimizing specific industrial applications of lasers. The main applications of industrial lasers are presented, as well as those of the rapidly emerging ultrafast laser family.