_applied_  
_sciences_  
_Editorial_  
Science at X-ray Free Electron Lasers  
**[PERSON] 1,2**  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
X-ray Free Electron Lasers (FELs) deliver coherent X-ray pulses, combining unprecedented power densities of up to 1020 W/cm\({}^{2}\) and extremely short pulse durations down to hundreds of attoseconds. Such intense X-ray FEL pulses make single-shot diffraction of nanometer-size objects, tiny protein crystals, and non-crystallized biomolecules a tangible reality. Such ultrashort X-ray FEL pulses allow us also to visualize femtosecond-scale temporal variations of charge and structure that may occur upon photoexcitation in any form of matter. On the other hand, since the X-ray FEL pulses give access to a new regime of X-ray intensities, they open new venues in studying the interaction between intense X-rays and various forms of matter. Understanding the ultrafast reactions induced by the X-ray FEL pulses is of fundamental interest, as well as of crucial importance, for the X-ray FEL applications. There are seven short-wavelength FEL facilities in operation in the world. The first FEL facility FLASH (the Free Electron LASer in Hamburg) in Germany started operation for users in 2005. It provides FEL pulses in the range of extreme ultraviolet to soft X-rays. The first hard X-ray FEL facility LCLS (the Linac Coherent Light Source) in the United States of America started user operation in 2009. In 2012, the second hard X-ray FEL SACLA (the Spring-8 Angstrom Compact free electron LASer) in Japan and the first fully coherent seeded FEL FERMI (the Free Electron laser Radiation for Multidisciplinary Investigations) in Italy started user operation. In 2017, PAL-XFEL (the Pohang Accelerator Laboratory X-ray Free Electron Laser) in Korea, European XFEL (European X-ray Free Electron Laser), SwissFEL (Swiss X-ray Free Electron Laser), and DCLS (the Dalian Coherent Light Source) in China started operations. Following the success of the first Special Issue _X-ray free electron laser_, we have edited the second Special Issue _Science at X-ray free electron lasers_. This Special Issue aims to cover recent developments of XFELs and sciences there, focusing on new beamlines, end stations, and operating modes, as well as relevant theoretical studies. For this purpose, this special issue collected the following articles. Where the variable gap undulators were installed, FLASH2 has been run simultaneously with FLASH1 since 2016. Here, [PERSON] and coworkers successfully reinstalled Reaction Microscope, which allows us to record three-dimensional momenta of all electrons and ions ejected from a single atom or molecule simultaneously, as descried by [PERSON] et al. [1] together with some beautiful science cases studied by this end station. At European XFEL, the first high-repetition-rate Hard X-ray FEL in the world, [PERSON] and coworkers successfully installed femtosecond X-ray experiments (FXE) instrument that allow us to study various kind of ultrafast X-ray photochemistry, as described by [PERSON] et al. [2], whereas [PERSON] and coworkers successfully installed the single particles, clusters, and biomolecules and serial femtosecond crystallography (SPB/SFX) instrument with a wide range of options of sample deliveries, which allow us to perform various class of structural biology experiments, as described by [PERSON] et al. [3] together with some beautiful showcase examples. [PERSON] and coworkers have been working on the installation of an X-ray pump and X-ray probe system in the soft X-ray SASE3 beamline, as an upgrade project, and [PERSON] et al. describes this project in ref. [4]. At SACLA, the second-oldest hard X-ray FE in the world, there are a few new instrumentations and developments to be noted. [PERSON] et al.