  We present a new high precision measurement of parity violation in the weak
interaction, using polarized muon decay. The TWIST collaboration has measured
$P_\mu^\pi \xi$, where $P_\mu^\pi$ is the polarization of the muon in pion
decay and $\xi$ describes the intrinsic asymmetry in muon decay. We find
$P_\mu^\pi \xi = 1.00084 \pm
0.00029\,(\textrm{stat.})_{-0.00063}^{+0.00165}\,(\textrm{syst.})$, in good
agreement with the standard model prediction of $P_\mu^\pi=\xi=1$. Our result
is a factor of 7 more precise than the pre-TWIST value, setting new limits in
left-right symmetric electroweak extensions to the standard model.
