  The BABAR collaboration has demonstrated the first application of the new Tau
Polarimetry technique. This polarimetry technique exploits the kinematic
coupling of $\tau$ decay products to the spin states of the $\tau$ and initial
state electron, to precisely determine the average beam polarization in an
$e^+e^-$ collider. The Tau Polarimetry technique is expected to be used at
Belle II following an upgrade to a polarized electron beam, where the precision
with which the average beam polarization is known is expected to be the
dominant systematic uncertainty in proposed future measurements which require
polarized beams. Applying Tau Polarimetry, BABAR has preliminarily measured the
PEP-II average beam polarization to be $\langle
P\rangle=-0.0010\pm0.0036_{\textrm{stat}}\pm0.0030_{\textrm{sys}}$.
