Transitions from the innermost shells of iron, especially
the K- and L-shelllines, provide a powerful tool for
probing the physical characteristics of hot plasmas in X-ray
sources. Their strength and purity allow important
conclusions to be drawn even with modest energy resolution.
They should also help in studying the regions around black
holes and neutron stars. In this book the state of the art
and themost recent theoretical and experimental
observations are presented. The book will be a valuable
source for future satellite missions. It addresses both
researchers and graduate students in astrophysics.