  In the two parts of this contribution we describe two related XMM-Newton
programs. The first part summarizes our study of the X-ray spectral properties
and variability of z>4 quasars (Shemmer et al. 2005). The second part presents
preliminary results from our ongoing XMM-Newton program to investigate the
X-ray spectral properties and variability of luminous, high accretion-rate
quasars at z~2--3. We find that the X-ray photon index does not depend on
luminosity or redshift, and there is suggestive evidence that it may depend on
the accretion rate. None of our quasars is significantly absorbed, and none
shows signatures of reflection. By jointly fitting high-quality spectra of
eight radio-quiet z>4 quasars, including three from our XMM-Newton
observations, we place tight constraints on the mean X-ray spectral properties
of such sources. Most of our quasars are significantly X-ray variable on
timescales of months--years, but none shows rapid (~1 hr timescale) variations.
