Krill oil is an extract prepared from a species of Antarctic krill, Euphausia superba. Two of the most important components in krill oil are omega-3 fatty acids similar to those in fish oil, and phospholipid-derived fatty acids (PLFA), mainly phosphatidylcholine (alternatively referred to as marine lecithin).

Studies have shown toxic residues in Antarctic krill and fish; however, the United States Food and Drug Administration has accepted notices from krill oil manufacturers declaring that krill oil and products derived from it meet the standards for Generally Recognized as Safe (GRAS) status, although the FDA itself has not tested the products.
While not an endangered species, Antarctic krill are a mainstay of the diets of many ocean-based species including whales and there is some environmental and scientific concern that their population has decreased dramatically both due to climate change and human harvesting.

Krill oil and oceanic fish oil are rich in omega-3 fatty acids, mainly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). While both contain some EPA and DHA as free fatty acids, krill oil contains particularly rich amounts of choline-containing phospholipids and a phosphatidylcholine concentration of 34 grams per 100 grams of oil.
Krill oil also contains appreciable content of astaxanthin at 0.1 to 1.5 mg/mL depending on processing methods, and is responsible for its red color.