Publisher Summary This chapter discusses the types of investigations with major emphasis on recent studies that include elucidation of complete or partial amino acid sequences of the enzymes from several sources. L-Glutamate dehydrogenases catalyze the interconversion of α-ketoglutarate and L-glutamic acid. The enzymes are recognized to be important because of the pivotal positions in metabolism occupied by glutamate and α -ketoglutarate and the ability of these compounds to enter into many types of pathways. Glutamate dehydrogenases provide a route for incorporation of nitrogen into organic compounds, and thus a link between carbohydrate and amino acid metabolism. There are at least three types of glutamate dehydrogenases which differ in coenzyme specificity. Those specific for either NAD or NADP and those that can function with both. The successful isolation from some species of modified forms of the enzyme produced by mutant strains, particularly of Neurospora, now permits identification of residues important for maintenance of normal activity.