  Gas accretion is necessary to maintain star formation, spiral and bar
structure, and secular evolution in galaxies. This can occur through tidal
interaction, or mass accretion from cosmic filaments. Different processes will
be reviewed to drive gas towards galaxy centers and trigger starbursts and AGN.
The efficiency of these dynamical processes can be estimated through
simulations and checked by observations at different redshift, across the
Hubble time. Large progress has been made on galaxies at moderate and high
redshifts, allowing to interpret the star formation history and star formation
efficiency as a function of gas content, dynamical state and galaxy evolution.
