Glutamate synthase is a multicomponent ironsulfur flavoprotein belonging to the class of N-terminal nucleophile amidotransferases. It catalyzes the conversion of L-glutamine and 2-oxoglutarate into two molecules of L-glutamate. In recent years the X-ray structures of the ferredoxin-dependent glutamate synthase and of the a subunit of the NADPH-dependent glutamate synthase have become available. Thanks to X-ray crystallography, it is now known that the ammonia reaction intermediate is CMLS, Cell. Mol. Life Sci. 61 (2004) 669–681 1420-682X/04/060669-13 DOI 10.1007/s00018-003-3316-0 © Birkhäuser Verlag, Basel, 2004 CMLS Cellular and Molecular Life Sciences transferred via an intramolecular tunnel from the amidotransferase domain to the synthase domain over a distance of about 32 Å. Although ammonia channeling is a recurrent theme for N-terminal nucleophile and triad-type amidotransferases, the molecular mechanisms of ammonia transfer and its control are different for each known amidotransferase. This review focuses on the intriguing mechanism of action and self-regulation of glutamate synthase with a special focus on the structural data.