The chapter provides an overview of cellulose biosynthesis and function in bacteria, focusing on the gram-negative bacterium *Acetobacter xylinum*. It highlights the highly ordered structure of cellulose fibrils and the unique mechanisms involved in their biogenesis. The chapter details the steps from glucose polymerization to the assembly of crystalline cellulose fibrils, emphasizing the role of cellulose synthase and the regulatory mechanisms, particularly the allosteric effector c-di-GMP. The text also discusses the comparative biochemistry of cellulose synthesis in various organisms, including plants, algae, and other bacteria, and explores the biological functions and genetics of bacterial cellulose. Finally, it touches on the biotechnological applications of bacterial cellulose, such as in the production of commercial products.The chapter provides an overview of cellulose biosynthesis and function in bacteria, focusing on the gram-negative bacterium *Acetobacter xylinum*. It highlights the highly ordered structure of cellulose fibrils and the unique mechanisms involved in their biogenesis. The chapter details the steps from glucose polymerization to the assembly of crystalline cellulose fibrils, emphasizing the role of cellulose synthase and the regulatory mechanisms, particularly the allosteric effector c-di-GMP. The text also discusses the comparative biochemistry of cellulose synthesis in various organisms, including plants, algae, and other bacteria, and explores the biological functions and genetics of bacterial cellulose. Finally, it touches on the biotechnological applications of bacterial cellulose, such as in the production of commercial products.