  Can the present dynamics of spiral galaxies tell us something about the
merging history, the formation and evolution of disks? Galaxy interactions
thicken or destroy disks; the simultaneous presence of thick and thin disks is
a tracer of past accretion, and also of recent disk re-formation. Observation
of a large number of counter-rotating disks is also evidence of past mergers,
as well as the frequency of polar-ring galaxies. Finally, the ubiquitous
presence of warps in the outer parts of HI disks might also provide a clue of
how frequently disks accrete mass with different angular momentum.
