Precision X-ray spectroscopy of one electron and one vacancy ions in the mid-to-high-Z region is discussed and results are compared to recent theoretical calculations. The comparisons are made as a function of Z and reveal deficiencies in the theoretical calculations and experimental measurements. Pertinent experimental topics are briefly covered including wavelength normalization, production of clean X-ray spectra (free of satellites), and high resolution instrumentation. Discussion of the theoretical calculations includes the variation of various terms as a function of Z and the importance of correlation effects in one vacancy ions. Future experimental measurements are briefly mentioned.