  I describe how the dynamics of galactic disks can be inferred by imaging and
spectroscopy. Next I demonstrate that the decomposition of the rotation curves
of spiral galaxies into the contributions by the various components of the
galaxies is highly degenerate. Constraints on the decomposition can be found by
considering implications for the dynamics of the galactic disks. An important
diagnostic is the Toomre Q stability parameter which controls the stability of
a galactic disk against local Jeans collapse. I also show how the density wave
theory of galactic spiral arms can be employed to constrain the mass of a
galactic disk. Applying both diagnostics to the example of NGC 2985 and
discussing also the implied mass-to-light ratio I demonstrate that the inner
parts of the galaxy, where the optical disk resides, are dominated by baryons.
When I apply this method to the disks of low surface brightness galaxies, I
find unexpectedly high mass-to light ratios. These could be explained by
population synthesis models which assume a bottom heavy initial mass function
similar to the recently proposed `integrated galactic initial mass function'.
