Thin layers are congregations of phytoplankton and zooplankton in the water column which were discovered with advances in instrumentation and deployment technologies allowed samples at the temporal and spatial scales where patterns were revealed. Although they may extend for kilometers, thin layers are only a few tens of centimeters in vertical thickness. They have distinct physical, biological, chemical, optical and acoustical signatures. Thin layers of phytoplankton or zooplankton may contain densities of organisms ranging up to 1000 times those found just above, or below the structure. These extraordinary concentrations of living material must have important implications for many aspects of marine ecology (e.g., phytoplankton growth dynamics, micro- and macrozooplankton grazing, behavio