The EUV spectral region contains a wealth of plasma diagnostics for stellar coronae (Te∼5×105 to 2×107 K). Of particular importance for understanding coronal structure are the observable emission lines of highly ionized iron (Fe VIII—XXIV), which allow the determination of electron temperatures (and the detailed temperature distributions) and electron densities. Comparison of continuum emission and lines from other elements with the iron lines provides diagnostics for relative abundances in the stellar atmospheres. Recent work in both solar and stellar coronal physics has greatly changed our picture of the corona, with EUV spectroscopy providing critical pieces of the puzzle. Here we discuss some important new spectral diagnostic results, examining in particular the quality of the theoretical atomic physics used in the data interpretation.