  Two major X-ray satellites, AXAF & XMM, will be launched in 1998/1999. Both
satellites have spectral and imaging capabilities, and are several times more
sensitive than either ROSAT or ASCA. AXAF & XMM will yield significant
scientific returns in many fields of astrophysics. We concentrate here on how
AXAF & XMM will improve constraints on the density parameter, Omega_0, via
measurements of the cluster abundance at high redshift.
