A molecular genetic approach using Arabidopsis and the availability of mutants with alterations in lipid composition have been instrumental in at least three areas. These mutants have facilitated the characterization of enzymes involved in lipid metabolism which, because many of them are integral membrane proteins, have been recalcitrant to characterization by traditional biochemical approaches. The mutants have also been used as markers to facilitate the cloning of genes that encode these enzymes by map-based chromosome walking and T-DNA tagging. Finally the availability of a series of plant lines with specific changes in lipid composition have provided a new resource to study the structural and adaptive roles of lipids.