  We derive the atomic hydrogen content for a total of 1900 spirals in the
fields of eighteen nearby clusters. By comparing the HI deficiency
distributions of the galaxies inside of and outside of one Abell radius of each
cluster, we find that two thirds of the clusters in our sample show a dearth of
neutral gas in their interiors. Possible connections between the gaseous
deficiency and the characteristics of both the underlying galaxies and their
environment are investigated in order to gain insight into the mechanisms
responsible for HI depletion. While we do not find a statistically significant
variation of the fraction of HI-deficient spirals in a cluster with its global
properties, a number of correlations emerge that argue in favor of the
interplay between spiral disks and their environment. In the clusters in which
neutral gas deficiency is pronounced, we see clear indications that the degree
of HI depletion is related to the morphology of the galaxies and not to their
optical size: early-type and, probably, dwarf spirals are more easily emptied
of gas than the intermediate Sbc--Sc types. The wealth of 21-cm data collected
for the Virgo region has made it possible to study the 2D pattern of HI
deficiency in that cluster. We also find evidence that gas-poor spirals in
HI-deficient clusters move on orbits more radial than those of the gas-rich
objects. The implications of all these results on models of how galaxies
interact with their environment are reviewed. Hydrodynamic effects appear as
the most plausible cause of HI removal. (abridged)
