Fatty acid desaturases play important roles in controlling the physical properties o f membranes and in the synthesis of signal molecules such as prostaglandins and pheromones. Most desaturases are membrane proteins that have been recalcitrant to characterization by conventional biochemical methods. Only one enzyme o f this class has been characterized from animals or fungi. In this context, plants have proved to be useful sources of experimental materials. Substantial progress has been made in characterizing and manipulating nine classes of desaturases that control the fatty acid composition o f both plant membranes and plant storage lipids, which account for approximately -30% of the calories in the human diet.