  The spin asymmetry parameter $A_\tau$ characterizing the angular distribution
of the total hadron momentum in the decay of a polarized tau can be calculated
rigorously using perturbative QCD and the operator product expansion.
Perturbative QCD corrections to the free quark result $A_\tau = 1/3$ can be
expressed as a power series in $\alpha_s(M_\tau)$ and nonperturbative QCD
corrections can be expanded systematically in powers of $1/M_\tau^2$. The QCD
prediction is $A_\tau = 0.41 \pm 0.02$. In the decay of a high energy tau into
hadrons, the value of the hadronic energy distribution $dR_\tau/dz$ evaluated
at the maximum hadronic energy fraction $z = 1$ can also be calculated
rigorously from QCD.
