  Recent surveys have discovered hundreds of low surface brightness galaxies in
the local (z < 0.1) Universe. Plots of the surface brightness distribution (the
space density of galaxies plotted against central surface brightness) show a
flat distribution from the bright-end cutoff of 21.65 through the current
observational limit of 25.0 B mag arcsec^{-2}. As no trend is seen to indicate
the size or mass of galaxies decreases with decreasing central surface
brightness, it is likely that a significant percentage of the baryon content in
the universe is contained in these diffuse systems. In this paper I briefly
review the known properties of low surface brightness galaxies, and describe
some current theories on the baryonic mass fraction of low surface brightness
systems and their consequences.
