Publisher Summary Dehydrins (DHNs) are a family of proteins that play a protective role in plants during cellular dehydration. Their production during the later stages of embryogenesis and in mature seeds, DHNs accumulate because of salinity, water deficit, low temperature, and in response to ABA treatment. DHNs can also be induced in response to changes in photoperiod. This chapter discusses the role of DHNs in abiotic stress adaptation in plants. Several in vitro studies have led to a consistent observation of cryoprotective properties of DHNs. Most of the evidence discusses the role of DHNs in stabilizing membranes and macromolecules, preventing structural damage during cellular dehydration, and maintaining the activity of essential enzymes. The biased amino acid composition of DHNs towards extreme hydrophilicity and their abundance argue in favor of a mass action rather than direct enzymatic role. This is supported by the absence of similarity to any known enzymes. Additionally, genetic evidence of a role of Dhn genes in natural variation in stress tolerance and in transgenic plants has started to accumulate, though none of the genetic evidence yet is entirely compelling.