  We study the longitudinal, transverse, and normal polarization components of
the tau lepton in the decays $\bar{B}^0 \to D^{(\ast)} \tau^- \bar\nu_{\tau}$
and discuss their role in searching for new physics (NP) beyond the standard
model (SM). Starting with a model-independent effective Hamiltonian including
non-SM four-Fermi operators, we obtain experimental constraints on different NP
scenarios and investigate their effects on the polarization observables. In the
SM the longitudinal and transverse polarizations of the tau lepton differ
substantially from the corresponding zero lepton mass values of $P_L=-1$ and
$P_T=0$. In addition, $P_L$ and $P_T$ are very sensitive to NP effects. For the
transverse polarization this holds true, in particular, for the effective
tensor operator in the case of $\bar{B}^0 \to D^\ast$ and for the scalar
operator in the case of $\bar{B}^0 \to D$. The $T$-odd normal polarization
$P_N$, which is predicted to be negligibly small in the SM, can be very sizable
assuming NP complex Wilson coefficients. We also discuss in some detail how the
three polarization components of the tau lepton can be measured with the help
of its subsequent leptonic and semihadronic decays.
