This article is available online at http://www.jlr.org ( 10 ), and regulation of energy homeostasis ( 11,12 ). atRA exerts its actions by serving as an activating ligand of nuclear atRA receptors [retinoid acid receptor (RAR) ␣ , RAR  , and RAR ␥ ] and peroxisome proliferator-activated receptors (PPAR)  / ␦ , which form heterodimers with retinoid X receptors ( 13,14 ). The actions of the RARs are described in more detail in this thematic series by RochetteEgly and colleagues. The concentration of atRA during embryonic development is tightly controlled in a spatial and temporal manner, and in adult tissues, it is maintained within a very narrow range that is specifi c for each given tissue. If the control mechanisms fail and the concentration of atRA exceeds or falls below the optimal range, tissues and cells undergo pathophysiological changes that in most severe cases can lead to disease. Thus, the maintenance of optimal atRA levels is essential for life. This review focusses on recent fi ndings regarding the mechanisms that control atRA homeostasis, with specifi c emphasis on the enzymes involved in atRA biosynthesis and degradation. All-trans -retinoic acid (atRA) is a highly potent derivative of vitamin A that is required for virtually all essential physiological processes and functions because of its involvement in transcriptional regulation of over 530 different genes ( 1, 2 ). For example, atRA is necessary for differentiation and development of fetal and adult tissues, stem cell differentiation, and apoptosis ( 3-7 ), and for support of reproductive functions ( 8, 9 ), immune response