Oilseed crops are major sources of oils for human nutrition, and an increasing proportion is also being utilized for industrial purposes. Recent advances in our understanding of the basic biochemistry of seed oil biosynthesis, coupled with identification of genes for oilseed modification, have set the stage for the genetic engineering of oilseed crops that produce ‘designer’ plant seed oils tailored for specific applications. In this review we provide an overview of seed oil biosynthesis and highlight the enzymatic steps that have already been targeted for genetic manipulation, with the end goal of producing seed oils containing desired amounts of fatty acid components. Furthermore, we describe the identification of genes from various wild plant species that are capable of producing structurally diverse fatty acids, and how these advances open the door to the production of entirely novel oils in conventional oilseed crops. Transgenic oilseeds producing high amounts of these novel fatty acids represent renewable sources of raw materials that may compete with, and eventually replace, some petrochemicals that are derived from non-renewable crude oil.