Several different omega-3s exist, but the majority of scientific research focuses on three: alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).

The human body can only form carbon–carbon double bonds after the ninth carbon from the methyl end of a fatty acid. Therefore, ALA and linoleic acid are considered essential fatty acids, meaning that they must be obtained from the diet. ALA can be converted into EPA and then to DHA, but the conversion (which occurs primarily in the liver) is very limited (Less than 15%). Therefore, consuming EPA and DHA directly from foods and/or dietary supplements is the only practical way to increase levels of these fatty acids in the body.

ALA is present in plant oils, such as flaxseed, soybean, and canola oils. DHA and EPA are present in fish, fish oils, and krill oils, but they are originally synthesized by microalgae at the base of the marine food chain, not by the fish. As microalgae move up the food chain, fish acquire the omega-3s and accumulate them in their tissues.

Omega-3s play important roles in the body as components of the phospholipids that form the structures of cell membranes.