  An unexpected characteristic of low surface brightness galaxies (LSBGs) is
that a significant number are massive and possess substantial amounts of atomic
gas. We present preliminary results of an ongoing program to obtain BVRIJHK
imaging, along with some nuclear spectroscopy, of a well-defined sample of
LSBGs which are gas-rich and of similar size to giant, high surface brightness
spiral galaxies (HSBGs). These LSBGs span the entire range of Hubble disk
morphologies. While their disks are bluer, on average, than comparable HSBGs,
the optical morphology of massive LSBGs indicates that many of these systems
have undergone previous star formation episodes. They typically have long disk
scale lengths, and range from $M_B = -16 to -22 (H_0 = 75 km/s Mpc^{-1})$.
About half of the LSBGs with bulges show evidence of nuclear activity, and
$\sim 30%$ have appear to be barred. These massive, gas-rich LSBGs apparently
have varied, and often complex, evolutionary histories.
