X-ray spectral diagnostics of luminous black holes and neutron stars is 
a powerful tool for probing the properties  of radiatively efficient 
accretion systems (Galactic X-ray binaries  and supermassive black holes 
in extragalactic AGN), and material in between.   I will discuss how 
high resolution X-ray spectra can be used to facilitate detailed studies 
of accretion systems (geometry, plasma conditions, dynamics:winds, 
accretion, BH spin), enable searches for the baryonic interstellar and 
intergalactic medium, and determine dust composition and distribution. 
Select projects related to the aforementioned topics will be discussed.