X-ray spectroscopy of astrophysical and laboratory plasmas has proven to be a very useful diagnostic tool.  With the increasing number of X-ray telescopes on astrophysical satellites (e.g. XMM-NEWTON, CHANDRA), and X-ray spectrometers attached to laboratory fusion experiments, there has been a recent interest in using X-ray wavelength diagnostics, leading to a range of scientific discoveries. I will present an introduction to X-ray spectroscopy, describing some of the potential spectroscopic diagnostic tools and the underlying atomic physics that is required to interpret such diagnostics.  I will then describe a long-standing discrepancy in the interpretation of X-ray emission from Fe16+ in solar, stellar and laboratory fusion plasmas. Recent work, using high quality atomic data along with appropriate collisional-radiative modeling, has helped to resolved this discrepancy.