Water absorbs substantially more light than does air and does so with a strong spectral bias. Dissolved organic materials add further to attenuation, again with a strong spectral bias. Water also suspends particles more effectively than does air, so light scattering also changes the quantity, polarization and spectral character of underwater light fields from what one experiences in air. How light propagates in turn determines how deeply the oceans are heated and to what depths primary production is possible. The quantity and spectral quality of light often limit primary production and influence the results of visual predation further up the food web as well as UV damage to DNA of near-surface plankton. Understanding light propagation is essential to in situ and remote sensing of temperature, salinity and phytoplankton standing crops and dynamics. UMaine has an unusual concentration of scientists skilled in various aspects and applications of marine optics.