  Galaxy interactions produce strong torques that generate radial gas flows.
There are two opposite tendencies of these flows, regarding abundance
gradients: homogeneization and gradient flattening, and enhanced star formation
in the center, and gradient steepening. Mechanisms to create abundance
gradients are discussed, and comparison with observations is detailed,
concerning spirals (such as collisional rings, starbursts..) or ellipticals
(formed or not in mergers). A review of N-body simulations predictions, taken
into account star formation, is also presented.
