  We study the nonperturbative functions in the Collins--Soper--Sterman
resummation formalism by examing Drell--Yan data in both fixed target and
collider experiments and then predict the transverse momentum distributions of
the $W^\pm$ and $Z^0$ bosons at the Tevatron. Our results differ from that in
the literature and agree better with published CDF data. Using statistical
arguments, we find a $1\,$fb${}^{-1}$ luminosity at the Fermilab Tevatron
should be able to provide useful constraints on the nonperturbative functions.
