This volume contains a comprehensive treatment of X-ray spectroscopy, as applied in astrophysics. It is presented in the form of extensive notes of lectures given by seven distinguished scientists at the Tenth Summer School of the European Astrophysics Doctoral Network. The subjects covered are: basic line and continuum radiation processes in X-ray and gamma-ray astronomy; atomic physics of collision- and radiation-dominated plasmas; X-ray spectroscopic observations with ASCA and BeppoSAX; future X-ray spectroscopy missions; X-ray optics, and X-ray spectroscopy instrumentation. The book, which will appeal to both researchers and graduate students, is timely in view of the scheduled launches of the big X-ray observatories AXAF and XMM in 1999.