The accumulation of compatible solutes, such as betaines, proline and sugar alcohols, is a widespread response that may protect plants against environmental stress. It is not yet fully understood how these compounds are involved in the stress tolerance of whole plants. Some plants have been genetically engineered to express enzymes that catalyze the synthesis of various compatible solutes. Some interventions have increased the tolerance of some crop plants to abiotic stress. Furthermore, analysis of such transgenic plants has begun to clarify the roles of compatible solutes in stress tolerance.