When you trace the origins of natural phenomena and life activities, many ultimately come down to how atoms and molecules are arranged and move, and even to the motion of electrons within their assemblies. If we could directly observe the arrangement and motion of atoms and molecules, and the motion of electrons, it is expected to greatly help improve our lives—for example, by elucidating the causes of intractable diseases and creating medicines, and by establishing methods to suppress substances that worsen the global environment. X-rays, familiar from hospital diagnostics, were discovered by Röntgen in 1895. Because they have a much shorter wavelength than visible light, they have been used to observe fine structures of matter at the atomic and molecular level. SPring-8 is the world’s most powerful X-ray source, but even so, its intensity is insufficient to observe instantaneous atomic and molecular motion. Lasers are light sources that emit extremely intense light. If an X-ray laser could be realized, it would be possible to observe instantaneous atomic and molecular motion. Lasers generate coherent light with aligned phase (light in which peaks align with peaks and troughs align with troughs) and are applied to various optical technologies, but it was impossible to create a short-wavelength X-ray laser as an extension of conventional laser technology. As a method to create a laser in the X-ray region, a scheme was proposed that does not use conventional light emission phenomena in matter, but instead controls and drives electrons in a high-energy accelerator and uses the light emitted from them. Because an X-ray laser is created using free electrons stripped from atoms, it is called an X-ray Free Electron Laser (X-ray Free Electron Laser: XFEL). This XFEL makes it possible to observe instantaneous atomic and molecular motion.