The chapter discusses the desiccation tolerance of prokaryotes, focusing on the physiological and biophysical mechanisms that enable bacteria to survive air-drying conditions. It highlights the importance of water in cellular processes and the role of water in the structure and function of proteins, DNA, and lipids. The text covers the properties of water, including its high heat of vaporization, surface tension, and osmotic pressure, and how these properties affect cellular hydration and dehydration. The chapter also explores the concept of chemical potential, osmotic pressure, and water potential, and how these factors influence the behavior of cells under different moisture conditions. It discusses the role of compatible solutes, such as sugars and amino acids, in stabilizing proteins and protecting cells from desiccation. The text further examines the dynamics of water inside and outside cells, the distribution of water within bacterial compartments, and the impact of water content on cellular function. Finally, it reviews methods for removing water from cells and the preferential exclusion hypothesis, which explains how compatible solutes can stabilize proteins by preferentially excluding water molecules from their surfaces.The chapter discusses the desiccation tolerance of prokaryotes, focusing on the physiological and biophysical mechanisms that enable bacteria to survive air-drying conditions. It highlights the importance of water in cellular processes and the role of water in the structure and function of proteins, DNA, and lipids. The text covers the properties of water, including its high heat of vaporization, surface tension, and osmotic pressure, and how these properties affect cellular hydration and dehydration. The chapter also explores the concept of chemical potential, osmotic pressure, and water potential, and how these factors influence the behavior of cells under different moisture conditions. It discusses the role of compatible solutes, such as sugars and amino acids, in stabilizing proteins and protecting cells from desiccation. The text further examines the dynamics of water inside and outside cells, the distribution of water within bacterial compartments, and the impact of water content on cellular function. Finally, it reviews methods for removing water from cells and the preferential exclusion hypothesis, which explains how compatible solutes can stabilize proteins by preferentially excluding water molecules from their surfaces.