The discovery of glutamate synthases (E.C. 1.4.1.13 and E.C. 1.4.7.1) caused a major re-assessment of the way in which ammonium is assimilated in bacteria and higher plants. The history of that discovery is reviewed and considered in the light of recent developments in the biochemistry and genetics of the higher plant ferredoxin- and NADH-dependent enzymes. The evidence is consistent with the view that plants synthesise glutamate from ammonia by the combined activity of glutamine synthetase (E.C. 6.3.1.2) and glutamate synthase (the glutamate synthase cycle) and that glutamate dehydrogenase (GDH, E.C. 1.4.1.2) plays no significant part in glutamate formation. The evidence does, however, suggest an important role for GDH as a catabolic shunt to ensure N metabolism does not adversely affect mitochondrial function and to enable the synthesis of N-rich transport compounds during nitrogen remobilisation.