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.