Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.

Krill are considered an important trophic level connection – near the bottom of the food chain. They feed on phytoplankton and (to a lesser extent) zooplankton, yet also are the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, seals, penguins, seabirds, squid, and fish each year. Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.

Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea. Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry. In Japan, the Philippines, and Russia, krill are also used for human consumption and are known as okiami (オキアミ) in Japan. They are eaten as camarones in Spain and Philippines. In the Philippines, krill are also known as alamang and are used to make a salty paste called bagoóng.

Krill are also the main prey of baleen whales, including the blue whale.