[en] X-ray spectra of highly ionized elements are widely used in diagnosing high temperature plasmas from astrophysical as well as from laboratory sources. The emission lines in the X-ray region are produced primarily from two different types of transitions. One type involves the transitions 1s-2p in hydrogen-like ions, helium-like ions and their associated satellites. The second type involves the transitions 2 l - n l ' for n > 2. These transitions are sensitive to electron temperature, electron density and the ionization balance. High quality spectrometers in the X-ray region have instrumental line widths that in many cases are significantly less than the true line widths. As a result, it is quite convenient to use the physical information in the line profiles to determine additional properties of the plasma under investigation. In this paper results from recent experiments concerning solar plasmas as well as laboratory sources are described. 13 references, 13 figures