We propose to use ISO to measure the far-infrared spectral energy
distributions of a large sample of high redshift quasars.  These
quasars have been selected to all have a redshift between 2.0 and
3.0, the epoch at which the co-moving space density of quasars
peaks.  This will be the most complete far-infrared study of high
luminosity, high redshift quasars performed to date, and will be
used to compare the range in far-infrared-to-optical/UV luminosity
ratios in these objects to that seen in quasars studied at low
redshift with the IRAS.  These observations will give us a detailed
picture of the global variation of the infrared-to-optical/UV
luminosity ratio among high redshift quasars, and be fundamental for
our understanding of quasar as well as galaxy evolution.  This proposal
is part of a large ongoing project to measure the far-infrared,
near-infrared, and optical properties of a large sample of both low
and high redshift quasars.