The chapter discusses the physiological and genetic responses of bacteria to osmotic stress, focusing on the mechanisms by which organisms adapt to changes in external osmolarity. Osmotic pressure and potential are defined, and the role of compatible solutes in maintaining cellular function under osmotic stress is highlighted. The accumulation of compatible solutes, such as K⁺ ions, glutamate, glutamine, proline, and trehalose, is described, along with their transport systems and regulatory mechanisms. The chapter also explores osmoremedial mutations, which affect the stability of proteins in response to osmotic stress, and mutations that confer sensitivity to hyperosmotic stress. The importance of maintaining electrolyte balance and the role of osmoprotectants like proline in protecting cellular components are discussed. The chapter concludes with an overview of the complex regulatory networks involved in osmoregulation in bacteria, emphasizing the interplay between gene expression, transport systems, and cellular metabolism.The chapter discusses the physiological and genetic responses of bacteria to osmotic stress, focusing on the mechanisms by which organisms adapt to changes in external osmolarity. Osmotic pressure and potential are defined, and the role of compatible solutes in maintaining cellular function under osmotic stress is highlighted. The accumulation of compatible solutes, such as K⁺ ions, glutamate, glutamine, proline, and trehalose, is described, along with their transport systems and regulatory mechanisms. The chapter also explores osmoremedial mutations, which affect the stability of proteins in response to osmotic stress, and mutations that confer sensitivity to hyperosmotic stress. The importance of maintaining electrolyte balance and the role of osmoprotectants like proline in protecting cellular components are discussed. The chapter concludes with an overview of the complex regulatory networks involved in osmoregulation in bacteria, emphasizing the interplay between gene expression, transport systems, and cellular metabolism.