Diffuse Hot Gas Under X-Ray Absorption Line Spectroscopy Daniel Q. Wang, Yangsen Yao University of Massachusetts Amherst B524 LGRT Astronomy Dept. Amherst, MA 01003 We will review our study of diffuse hot gas, based on both Chandra and XMM-Newton grating spectroscopic observations. In particular, we will report the results from our recent 100 ksec TOO observation of LMC X-3. The OVII absorption line by the Galactic gas is clearly detected, with an equivalent width comparable to those seen in the spectra of AGNs. In addition, we have systematically analyzed archival data on both Galactic X-ray binaries and AGNs, which has led to the detection of various absorption lines (O~VII Kalaph, O~VII Kbeta and/or O~VIII~Kalpha). We have also conducted comparisons with diffuse X-ray emission and far UV OVI line observations to estimate or tightly constrain the spatial, kinematic, and thermal properties of the gas. We will also discuss the implications of the results for the understanding of the Galactic gaseous halo and the warm/hot intergalactic medium.