  Low Surface Brightness (LSB) galaxies form a large population of disc
galaxies that extend the Hubble sequence towards extreme late-types. They are
only slowly evolving, and still in an early evolutionary state. The
Tully-Fisher relation and rotation curves of LSB galaxies both show that LSB
galaxies are very dark-matter dominated with respect to ``normal'' high surface
brightness (HSB) galaxies. Mass models derived from the rotation curves of LSB
and HSB galaxies show that LSB galaxies inhabit less dense and more extended
halos. Mass density, which changes with surface brightness, is as important in
determining the evolution of a galaxy as total mass is.
