Glycinebetaine (GB) has been studied extensively as a compatible solute because of the availability of GB-accumulating transgenic plants that harbor a variety of transgenes for GB-biosynthetic enzymes. Both the exogenous application of GB and the genetically engineered biosynthesis of GB increase the tolerance of plants to abiotic stress. As reviewed here, studies of such increased tolerance to abiotic stress have led to considerable progress in the characterization of the roles of GB in stress tolerance in plants. In particular, the reproductive organs of GB-accumulating transgenic plants exhibit enhanced tolerance to abiotic stress. Furthermore, accumulation of GB results in increased yield potentials under non-stress conditions.