Dehydration tolerance (DT) is defined as the ability of an organism or tissue to survive the removal of all, or almost all the cellular water without irreversible damage. DT of seeds is an adaptive mechanism to ensure the survival and reproduction of plant species in the long-term evolution process, and plays a key role in the conservation of plant seeds and germplasm resources. However, the DT of seeds is a complex trait, and its molecular mechanism is not now largely understood. Therefore, in the present paper, the research progresses on the physiological and molecular mechanisms of seed DT were reviewed. It was found that the DT of orthodox seeds was gradually formed during development, and reached the peak at physiological maturity. Recalcitrant seeds do not undergo the development stage of maturity dehydration, and are very sensitive to dehydration throughout development. Mature orthodox seeds maintained their resistance to re-dehydration at the initial stage of imbibition. With the time course of germination, the DT decreased gradually, and finally lost completely. The DT of seeds and embryos can be re-established during the early stage of germination, and of different tissues is different. The DT of seeds and embryos was inversely correlated with the decrease in mitochondrial respiratory activity. Respiratory activity of recalcitrant axis mitochondria was higher than that of orthodox embryo ones. During dehydration, the H₂O₂ content, the production rate of superoxide anion radical (·O₂^-) and the content of thiobarbituric acid reactive substance in desiccation-tolerant embryos (axes) were significantly lower than those of desiccation-sensitive embryos (axes), while the reactive oxygen species scavenging system in desiccation-tolerant embryos (axes), including enzymatic and non-enzymatic activities, was significantly higher than that in desiccation-sensitive embryos (axes). During the maturation of seeds, the accumulation of late embryogenesis abundant (LEA) proteins, small heat shock proteins and non-reducing oligosaccharides is closely related to the formation of DT. The AFL subfamily of B3 transcription factors (including ABI3 (ABA INSENSITIVE 3), FUS3 (FUSCA3) and LEC2 (LEAFY COTYLEDON 2)) increase the DT of seeds and embryos by positively regulating the accumulation of storage materials and protective proteins. The level of DNA methylation increased significantly throughout seed development and then decreased gradually during seed germination. Compared with embryos during the early stage of development and seedlings, mature embryos had a higher level of genomic methylation. In seeds, the parallel ABA and DOG1 (DELAY OF GERMINATION 1) signaling pathways activate synthesis of raffinose family oligosaccharides, and expression of LEA and HSP (heat shock protein) genes, thus regulating the onset of DT and transit to dormancy. Finally, the scientific issues that require to be further studied in this field are proposed, including the re-establishment of their model research system by using seeds and their tissues with different DT. Germinability, DT and dormancy characteristics of seeds are initiated and completed during development, and the relationship among them is still now unclear. There are both core ABA signaling pathway and DOG1 signaling pathway in seeds, and they converge at the ABI3 or downstream of ABI3. Which pathway will response preferentially and how these two pathways coordinate during dehydration of seeds? This paper will provide a reference for comprehensively understanding of the physiology and molecular mechanism of seed DT, increasing the stress resistance and yield of plant crops, improving the storage conditions of the resource bank and long-term preserving plant seed (germplasm) resources.