What can we learn from X-ray Spectroscopy?

X-ray spectroscopy and line diagnostics offer critical tools in X-ray Astrophysics. Taking advantage of the newly updated AtomDB and XStar, I will introduce the resent research of our team for a couple of applications in various scales of astrophysical plasmas based on the high and low spectral resolution observations from Chandra, XMM-Newton, Suzaku and HST/COS.