  The gas component plays a major role in the dynamics of spiral galaxies,
because of its dissipative character, and its ability to exchange angular
momentum with stars in the disk. Due to its small velocity dispersion, it
triggers gravitational instabilities, and the corresponding non-axisymmetric
patterns produce gravity torques, which mediate these angular momentum
exchanges. When a srong bar pattern develops with the same pattern speed all
over the disk, only gas inside corotation can flow towards the center. But
strong bars are not long lived in presence of gas, and multiple-speed spiral
patterns can develop between bar phases, and help the galaxy to accrete
external gas flowing from cosmic filaments. The gas is then intermittently
driven to the galaxy center, to form nuclear starbursts and fuel an active
nucleus. The various time-scales of these gaseous flows are described.
