The omega 3 polyunsaturated fatty acids have had a major impact on thinking in medicine in the last twenty years. The parent fatty acid in the omega 3 fatty acid family is alpha-linolenic acid (ALA) which is an essential fatty acid found in high concentrations in certain plant oils, such as flaxseed oil, walnut oil and canola oil. Several longer chain or derived omega 3 fatty acids are formed from alpha-linolenic acid and these are mainly found in fish, fish oils and from other marine organisms. The main marine omega 3 fatty acids are eicosapentaenoic acid (EPA), docosapentaenoic acid and docosahexaenoic acid (DHA). It is of interest that DHA is specifically localised in the retina and the brain in humans and other mammals. The longer chain omega 3 fatty acids are rapidly incorporated into cell membrane phospholipids where it is regarded they influence the metabolism/metabolic events within the cells. The mechanisms by which these changes occur include alteration in the fluidity of membranes such that there are subtle changes in receptor function, alteration in cell signalling mechanisms, membrane-bound enzymes, regulation of the synthesis of eicosanoids, and regulation of gene expression. In this chapter, we report a comparison between the composition of the oil derived from the New Zealand Green Lipped Mussel (Lyprinol') and two other oils rich in omega 3 fatty acids, namely flaxseed oil and tuna oil. The main lipid classes in Lyprinol' were sterol esters, triglycerides, free fatty acids, sterols and phospholipids while triglycerides were the main lipids in the other two oils. The main omega 3 fatty acids in Lyprinol' were EPA and DHA, while in flaxseed oil and tuna oil the main omega 3 fatty acids were ALA and DHA, respectively. The main sterols in Lyprinol' were cholesterol and desmosterol/brassicasterol, while in flaxseed oil and tuna oil the main sterols were beta-sitosterol and cholesterol, respectively. Epidemiological observations, populations' studies and basic research indicate the possibility of influencing the outcome of cardiovascular disease, inflammatory disorders and neural function by ingestion of the omega 3 polyunsaturated fatty acids.