  Non-axisymmetric modes like spirals and bars are the main driver of the
evolution of disks, in transferring angular momentum, and allowing mass
accretion. This evolution proceeds through self-regulation and feedback
mechanisms, such as bar destruction or weakening by a central mass
concentration, decoupling of a nuclear bar taking over the gas radial flows and
mass accretion, etc.. These internal mechanisms can also be triggered by
interaction with the environment. Recent problems are discussed, like the
influence of counter-rotation in the m=1 and m=2 patterns development and on
mass accretion by a central AGN.

Astrophysics