The chapter discusses the potential of using cellulose as an alternative energy source, particularly for ethanol production. It highlights the abundance and renewability of lignocellulose, which is the most abundant renewable natural resource. The text emphasizes the environmental and economic benefits of using cellulose to produce ethanol, including reduced greenhouse gas emissions and improved energy security. The focus is on the use of microorganisms, particularly anaerobic bacteria like *Clostridium thermocellum*, for the efficient breakdown of cellulose into sugars that can be converted to ethanol. The chapter also delves into the properties and mechanisms of the cellulase system in *C. thermocellum*, including the role of specific enzymes and the structure of the cellulosome, a complex aggregate of proteins that facilitates the hydrolysis of crystalline cellulose. The text concludes by discussing the advantages of direct fermentation of cellulose to ethanol, such as reduced processing costs and increased efficiency compared to traditional methods using fungi and yeast.The chapter discusses the potential of using cellulose as an alternative energy source, particularly for ethanol production. It highlights the abundance and renewability of lignocellulose, which is the most abundant renewable natural resource. The text emphasizes the environmental and economic benefits of using cellulose to produce ethanol, including reduced greenhouse gas emissions and improved energy security. The focus is on the use of microorganisms, particularly anaerobic bacteria like *Clostridium thermocellum*, for the efficient breakdown of cellulose into sugars that can be converted to ethanol. The chapter also delves into the properties and mechanisms of the cellulase system in *C. thermocellum*, including the role of specific enzymes and the structure of the cellulosome, a complex aggregate of proteins that facilitates the hydrolysis of crystalline cellulose. The text concludes by discussing the advantages of direct fermentation of cellulose to ethanol, such as reduced processing costs and increased efficiency compared to traditional methods using fungi and yeast.