  We study CP violation in semi-leptonic $\tau$ decays and we determine the
conditions necessary to be able to define a observable CP asymmetry. We apply
these conditions in both models, the standard model for the electroweak
interactions and its supersymmetric extensions. In the first case, the leading
order contribution to the direct CP asymmetry in $\tau^{\pm} \to K^{\pm}\pi^0
\nu_{\tau}$ decay rates is evaluated. In the second case,we compute the SUSY
effective hamiltonian that describes the $|\Delta S|=1$ semileptonic decays of
tau leptons. We show that SUSY contributions may enhance the CP asymmetry of
$\tau \to K \pi \nu_{\tau}$ decays by several orders of magnitude compared to
the standard model expectations.
