New light sources based on linear accelerators such as FLASH at DESY in Hamburg, the first free-electron laser in the extreme ultraviolet (XUV) and soft X-ray regime, which started user operation in 2005 [1], the Linac Coherent Light Source LCLS in Stanford [2], and SACLA at Spring-8 in Japan [3], as X-ray lasers dedicated to the hard X-ray regime down to below 1 Å in wavelength, or FERMI at ELETTRA in Trieste [4] as the first fully externally seeded free electron laser also operating in the XUV and soft X-ray regime, provide ultrashort, extremely powerful, short wavelength pulses with unprecedented coherence properties. With the European XFEL in Hamburg, the Swiss FEL at PSI in Villigen, Switzerland, and the PAL-XFEL in Pohang, Korea, three more FELs are expected to produce first light by the end of 2016 and the beginning of 2017, respectively.