Trehalose is a non-reducing disaccharide that consists of two glucose monomers bound by an a,a-1,1-glucoside bond. Trehalose is present in a wide range of organisms such as bacteria, fungi, invertebrates and plants. It has been demonstrated that trehalose stabilizes proteins and lipid membranes under various stress conditions, including heat stress, carbon starvation, osmotic or oxidative stress, etc. In most plants, trehalose levels are negligible, although there are many genes linked to trehalose biosynthesis. For instance, in Arabiospsis thaliana, there are 11 orthologues of the microbial trehalose-6-phosphate (T6P) synthase (TPS) family, divided in two classes, and there are 10 T6P phosphatases (TPP). TPS converts UDP-glucose and glucose-6-phosphate into T6P with the help of TPS enzymes and this T6P is hydrolyzed by TPPs to produce trehalose. In general, there are much less and often only one (as in A. thaliana) enzyme trehalase (TRE) that hydrolyzes trehalose into two glucose monomers. It has been shown that trehalose metabolism is critical for normal plant growth and development. Trehalose feeding causes growth arrest as stunted root growth and undeveloped primary leaves of seedlings. Modification of T6P levels by expression of either microbial TPS or TPP enzymes also results in aberrant growth phenotypes, which indicates that the level of T6P is very important for correct plant development. T6P levels also provide plants with information regarding the sugar status of the plants. Overexpression of both TPS and TPP or overexpression of active plant TPS enzymes result in small increases in trehalose levels and in an increased stress tolerance. However, the latter phenotype cannot be explained by the stress-protecting characteristics of trehalose as the concentrations are too low. Therefore, the hypothesis is that small changes in trehalose and/or T6P levels affect many plant characteristics, including growth, development and stress tolerance. All these aspects are discussed in this chapter with a focus on what is known in A. thaliana but also includes the recent findings in crop plants.