  Tau decays into hadrons foresee the study of the hadronization of vector and
axial-vector QCD currents, yielding relevant information on the dynamics of the
resonances entering into the processes. We analyse tau -> (3 pion) nu_tau
decays within the framework of the Resonance Chiral Theory, comparing this
theoretical scheme with the experimental data, namely ALEPH spectral function
and branching ratio. Hence we get values for the mass and on-shell width of the
a_1(1260) resonance, and provide the structure functions that have been
measured by OPAL and CLEO-II.
