  Low surface brightness regions are found not only in dwarf and ultra-diffuse
galaxies, but also on the outer parts of giant spirals, or in galaxy extensions
(tidal or ram-pressure tails, outflows or jets). Sometimes molecular gas is
detected in sufficient quantities to allow star formation, but the efficiency
is much lower than in disk galaxies.
  This presentation reviews different environments showing low-surface
brightness, their gas content and surface densities, and their star formation
efficiency. Some interpretations are proposed to account for this low
efficiency.
