Observation of a gravitational wave is the most spectacular recent application of a laser (published in Physical Review Letters, February 2016). Four Nobel Prizes in the last four years for achievements in physics and chemistry (see Sect. 1.9 of the book) demonstrate the signi?cance of lasers for scienti?c research. There is a steady development of lasers and of their use in scienti?c research in physics, chemistry, engineering, biophysics, medicine, and technical applications. Important progress has been made in the last years in the development and application of infrared and far-infraredfree-electronlasers,andofX-rayfree-electronlasers.X-rayfree-electron lasers are opening new possibilities in scienti?c research and in application.