Dehydration tolerance in plants is an important but understudied component of the complex phenotype of drought tolerance. Most plants have little capacity to tolerate dehydration; most die at leaf water potentials between -5 and -10 MPa. Some of the non-vascular plants and a small percentage (0.2%) of vascular plants, however, can survive dehydration to -100 MPa and beyond, and it is from studying such plants that we are starting to understand the components of dehydration tolerance in plants. In this chapter we define what dehydration tolerance is and how it can be assessed, important prerequisites to understanding the response of a plant to water loss. The metabolic and mechanical consequences of cellular dehydration in plants prelude a discussion on the role that gene expression responses play in tolerance mechanisms. We finally discuss the key biochemical aspects of tolerance focusing on the roles of carbohydrates, late embryogenesis abundant and heat shock proteins, reactive oxygen scavenging (ROS) pathways, and novel transcription factors. It is clear that we are making significant advances in our understanding of dehydration tolerance and the added stimulus of new model systems will speed our abilities to impact the search for new strategies to improve drought tolerance in major crops.