  Recent X-ray observations have had a major impact on topics ranging from
protostars to cosmology. They have also drawn attention to important and
general physical processes that currently limit our understanding of thermal
and nonthermal X-ray sources. These include unmeasured atomic astrophysics data
(wavelengths, oscillator strengths etc.), basic hydromagnetic processes (e.g.
shock structure, reconnection), plasma processes (such as electron-ion
equipartition and heat conduction) and radiative transfer (in disks and
accretion columns). Progress on these problems will probably come from
integrative studies that draw upon observations, throughout the electromagnetic
spectrum, of different classes of source. X-ray observations are also giving a
new perspective on astronomical subjects, like the nature of galactic nuclei
and the evolution of stellar populations. They are contributing to answering
central cosmological questions including the measurement of the matter content
of the universe, understanding its overall luminosity density, describing its
chemical evolution and locating the first luminous objects. X-ray astronomy has
a healthy future with several international space missions under construction
and in development.
