  We study the M_bh - M_host relation as a function of Cosmic Time in a sample
of 96 quasars from z=3 to the present epoch. In this paper we describe the
sample, the data sources and the new spectroscopic observations. We then
illustrate how we derive M_bh from single-epoch spectra, pointing out the
uncertainties in the procedure. In a companion paper, we address the dependence
of the ratio between the black hole mass and the host galaxy luminosity and
mass on Cosmic Time.
