Due to the high reduction level of the carbons in fatty acids, the oxidation of triacylglycerols (TAGs) releases much more energy than the oxidation of other storage compounds like carbohydrates or proteins. TAGs are synthesized in many animal and plant species to act as an energy reserve. Here, we review some of the studies that have contributed to decipher the metabolic and regulatory networks responsible for the biosynthesis of TAGs in two contrasted model systems: the liver of mice and the maturing oilseed of Arabidopsis. A comparison of the two systems illustrates how distinct transcriptional regulatory systems trigger lipogenesis in response to specific signals: a high carbohydrate diet induces TAG synthesis in the liver of mammalians whereas a developmental program initiates TAG accumulation at the onset of embryo maturation in Arabidopsis.