  Recently $f(T)$ modified teleparallel gravity where T is the torsion scalar
has been proposed as the natural gravitational alternative for dark energy. We
perform a detailed dynamical analysis of these models and find conditions for
the cosmological viability of $f(T)$ dark energy models as geometrical
constraints on the derivatives of these models. We show that in the phase space
exists two cosmologically viable trajectory which (i) The universe would start
from an unstable radiation point, then pass a saddle standard matter point
which is followed by accelerated expansion de sitter point. (ii) The universe
starts from a saddle radiation epoch, then falls onto the stable matter era and
the system can not evolve to the dark energy dominated epoch. Finally, for a
number of $f(T)$ dark energy models were proposed in the more literature, the
viability conditions are investigated.
