Transitions from the innermost shells of iron, especiallythe K- and L-shelllines, provide a powerful tool forprobing the physical characteristics of hot plasmas in X-raysources. Their strength and purity allow importantconclusions to be drawn even with modest energy resolution.They should also help in studying the regions around blackholes and neutron stars. In this book the state of the artand themost recent theoretical and experimentalobservations are presented. The book will be a valuablesource for future satellite missions. It addresses bothresearchers and graduate students in astrophysics.