Trehalose is a non-reducing disaccharide widely distributed in nature. To date, four enzymatic systems for its biosynthesis have been characterised. The most extensively studied is the UDP-glucose 6-phosphate dependent trehalose synthase/trehalose phosphatase system (TPS/TPP), which presents the unique property to be part of a multifunctional protein in Saccharomyces cerevisiae. The TreYZ, which generates trehalose from glycogen-like structures by a maltooligosyltrehalase (MOtase) and a maltooligosyl trehalose hydrolase (MOTHase), and the trehalose synthase (TreS), which catalyses the reversible conversion of maltose into trehalose, are two other enzymatic systems that have been identified in a few bacteria. A fourth potential mechanism is the formation of trehalose from glucose and glucose-1-phosphate catalysed by a trehalose phosphorylase. Athough this enzyme most likely functions in vivo in the degradation of trehalose, the main catabolic system of this disaccharide is its hydrolysis into glucose by trehalases. Most microbial cells are endowed with a cytoplasmic and an extracellular trehalases. The main role of the cytosolic enzyme is to regulate the intracellular trehalose concentration in response to stress and developmental conditions. In contrast, the extracellular trehalase is required for growth on trehalose. The uptake of trehalose mediated by a phosphotransferase system in bacteria and by a H⁺-trehalose symport represents another mean for trehalose assimilation. Because of its unique properties as a cell protectant and protein stabilizator, in vitro and in situ processes have been set up to increase the production of trehalose. However, deeper knowledge of these different metabolic systems and of their regulation should help further progress in biotechnological applications of this disaccharide