This article will focus on the modification of plant seed oils to enhance their nutritional composition. Such modifications will include C18 Δ6-desaturated fatty acids such as γ-linolenic and stearidonic acid, omega-6 long-chain polyunsaturated fatty acids such as arachidonic acid, as well as the omega-3 long-chain polyunsaturated fatty acids (often named ‘fish oils’) such as eicosapentaenoic acid and docosahexaenoic acid. We will consider how new technologies (such as synthetic biology, next-generation sequencing and lipidomics) can help speed up and direct the development of desired traits in transgenic oilseeds. We will also discuss how manipulating triacylglycerol structure can further enhance the nutritional value of ‘designer’ oils. We will also consider how advances in model systems have translated into crops and the potential end-users for such novel oils (e.g. aquaculture, animal feed, human nutrition).