  Except in the most extreme cases of nuclear activity, either starbursts or
AGN, it is difficult to find observationnally a close link between the dynamics
and the activity. Theoretically however, the necessary step to fuel the gas to
the center, is that gravity torques are created through a non-axisymmetric
pattern, either bar and/or spiral, triggered or not by a tidal interaction.
  We describe the sequence of processes for a typical evolution cycle for a
spiral galaxy, and the possible efficient feedback mechanisms. The various
morphologies and dynamical states of spiral galaxies are interpreted in terms
of a sequence of evolutionary phases, and the corresponding time-scales can be
estimated from observations. In this scenario, activity in galaxies is related
to the appearance of bar instability, although they might not be synchronised
in phase. The role of external gas accretion in the secular evolution is
discussed.
