Polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (EPA, 20:5n3) and docosahexaenoic acid (DHA, 22:6n3) are essential to human health and nutrition, providing important visual, mental, and cardiovascular benefits throughout life. One pathway for the biosynthesis of PUFAs relies on the alternating action of desaturases and elongases on fatty acid substrates. Alternatively, PUFA synthases can catalyze the de novo production of PUFAs from malonyl-CoA. These enzymes have been detected in both psychrophilic marine bacteria (such as Shewanella and Moritella) and in the eukaryotic thraustochytrids (such as Thraustochytrium and Schizochytrium). Much of our work has focused on the PUFA synthase expression in a species of Schizochytrium that has been developed as a commercial source for biomass and oil enriched in DHA. We will present data concerning the synthesis pathway and the accumulation of PUFAs in this organism. Additionally, the independence of PUFA synthases from the standard fatty acid synthesis pathways and the relatively pure end-products has made them attractive targets for heterologous expression. Data related to the genes encoding PUFA synthases, their expression in bacteria, yeast and higher plants, and strategies to manipulate PUFA profile will be presented.