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.