  I review briefly some dynamical models of structures in the outer parts of
disc galaxies, including models of polar rings, tidal tails and bridges. I then
discuss the density distribution in the outer parts of discs. For this, I
compare observations to results of a model in which the disc galaxy is in fact
the remnant of a major merger, and find good agreement. This comparison
includes radial profiles of the projected surface density and of stellar age,
as well as time evolution of the break radius and of the inner and outer disc
scale lengths. I also compare the radial projected surface density profiles of
dynamically motivated mono-age populations and find that, compared to older
populations, younger ones have flatter density profiles in the inner region and
steeper in the outer one. The break radius, however, does not vary with stellar
age, again in good agreement with observations.
