Based on the record of the oldest identifiable fossils, the first oxygenic photosynthetic organisms appeared about 2 X 10 to the 9th power years ago in the form of marine single-celled, planktonic prokaryotes. In the intervening eons, phytoplankton have evolved and diversified; presently they represent at least 11 classes of prokaryotic and eukaryotic photoautotrophs. While the carbon of these organisms cumulatively amounts to only 1-2% of the global plant biomass, they fix between 35 and 50 gigatonnes of carbon annually. This biomass converts approximately 0.13% of the photosynthetically active radiation (440-700 nm) incident on the sea surface to photochemical energy. Despite a great deal of variability in ocean environments, this photosynthetic conversion efficiency is relatively constant for integrated water column production. Here we review the factors determining light utilization efficiency of phytoplankton in the oceans, and the physiological acclimations which have evolved to optimize light utilization efficiency.