This chapter reviews the plant responses to salinity stress with emphasis on physiological, biochemical and molecular mechanisms of salt tolerance. Genes for enzymes involved in the biosynthesis and metabolism of proline have been isolated from various plants, and their expression and the functions of their gene products have been characterized. The core of proline metabolism involves two enzymes catalysing proline synthesis from glutamate in the cytoplasm or chloroplast, two enzymes catalysing proline catabolism back to glutamate in the mitochondria, as well as an alternative pathway of proline synthesis via ornithine. The proline accumulated in response to water stress or salinity stress in plants is primarily localized in the cytosol. In plants, under abiotic stress, low molecular weight osmoprotectants and osmolytes such as proline, glycine betaine and sugar alcohols are synthesized and accumulated, and their synthesis and degradation have been studied well.