The use of x rays has moved in the forefront of science and technology in the second half of the 20th century. This progress has been greatly stimulated by the advent of synchrotron x-ray sources in the 1960s. The undulator-based synchrotron radiation sources which have appeared in the last decade of the 20th century gave a new impetus to such development. The brilliance of the x-ray sources has increased by 12 orders of magnitude in 40 years and this trend does not show any signs of stagnation. The future x-ray sources of the 21th century based on free-electron lasers driven by linear accelerators will provide sub-picosecond radiation pulses with by many orders of magnitude higher brilliance and full transverse coherence. The x-ray sources of the newest generation offer a possibility to realize more than ever before the great potential of x-ray optics and, as a consequence, to elaborate new sophisticated instrumentation with unprecedented resolution and eventually to move in new directions of research in x-ray technology, materials science, fundamental physics, life sciences, etc.
