Marine invertebrates are a surprisingly rich source of oxidized fatty acid-derived substances known as oxylipins. They occur in many phyla of marine invertebrates, including Porifera, Coelenterata, Crustacea, Mollusca, Echinodermata, and Urochordata. Many of the oxylipins obtained from these life forms are structurally unique and have no direct parallel to oxylipins from other sources. Consequently, the biosynthetic pathways responsible for their formation are also unique. Even those oxylipins of comparable structure to those occurring terrestrially are produced by novel biosynthetic pathways in marine organisms. Hence, biosynthetic studies of marine oxylipins are providing fundamental new insights into the scope and nature of oxylipin chemistry and metabolism. Further, study of the ecological and potentially useful pharmacological properties of these unique substances continues to provide new opportunities in medicine and biology.