This chapter describes the biosynthesis and structure of fats as well as the mechanisms of fatty acid biosynthesis and degradation via β-oxidation. It begins with an explanation of the acetate rule, which influences the structure of many natural substances, including fatty acids and polyketides. The differences between fats and fatty oils as well as the chemical structural features of triglycerides are explained, highlighting the effects on consistency and properties. The importance of fatty acid biosynthesis and the β-oxidation pathway for energy metabolism is emphasized, including the utilization of fatty acids for regeneration of energy-rich molecules such as ATP and NAD(P)H. A dedicated section on ABE metabolism deals with the anaerobic fermentation of carbohydrates, which leads to the production of acetone, butanol, and ethanol. The role of clostridia, especially Clostridium acetobutylicum, and the significance of the acidogenic and solventogenic phases of metabolism are explained. Furthermore, the causes and effects of spore formation at different butanol concentrations and the technical importance of ABE metabolism for biobutanol production are discussed. Overall, this chapter provides a comprehensive overview of the structure, and metabolic pathways leading to fats, fatty acids, and related compounds, illuminating both the biological mechanisms and their technical applications.