Zooplankton is a term applied to the community of animals that live adrift in the water, with a limited capacity to counteract the movements of water by swimming. The complexity of taxonomic composition, morphological structure, size range and trophic role of zooplankton are probably unique in the marine world. The lack of rigid relations (i.e., space occupation) between these organisms living suspended in water is probably one of the main sources of zooplankton diversity. Taxonomically, zooplankton ranges from the most primitive unicellular organisms (protists) to vertebrates (fish larvae). While some species spend their entire life suspended in water, without any contact with solid surfaces (holoplankton), most bottom-living marine invertebrates and fishes, have also a transitory planktonic life, usually the first larval stages, and are known as meroplankton. Despite their quite limited swimming capacity, they perform night-day rhythmic vertical displacements of hundreds of meters, most of them to feed at night in surface waters. Their main food source is phytoplankton, microscopic plants that provide most of the matter and energy in marine systems, although their trophic mode is by no means rigid. Some are strict carnivores, but in general they can switch from vegetal to animal or even detritus food sources. This chapter briefly describes the role of zooplankton in the environment, its important feeding links, as well as the potential socio-economic consequences of shifts in its spatial distribution and time changes.