gamma -Aminobutyrate (GABA) is a non-proteinaceous amino acid which plays different roles in the living systems. GABA is biochemically produced by the irreversible a-decarboxylation of l-glutamate catalysed by glutamate decarboxylase (Gad), a widespread enzyme distributed among eukaryotes and prokaryotes. Structural features of the plant, mammalian and bacterial Gads are reported in the literature. In pathogenic bacteria, Gad activity has been linked to protection from acid stress, but the possibility cannot be excluded that it might perform other physiological roles. Indeed, the screening of lactic acid bacteria (LAB) based on their capacity to synthesize GABA opens a new perspective for the production of GABA containing dairy foods. The aim of this chapter is to provide an overview of the best characterized Gad enzymes in pathogenic and non-pathogenic bacteria, along with a description of the potential use of Gad as a source of GABA in functional foods and in biotechnological products.