This chapter examines catabolism as the central energy-releasing process in microbial metabolism, emphasizing its role in wastewater treatment. It explains how organic and inorganic substrates are broken down through glycolysis, the Krebs cycle, and oxidative phosphorylation to generate ATP, NADH, and FADH₂. The link between electron transfer, energy conservation, and substrate oxidation is explored in detail, illustrating how microbes harness these pathways under aerobic and anaerobic conditions. Special attention is given to the role of alternative electron acceptors such as nitrate in anoxic environments, highlighting their significance in engineered treatment systems. By quantifying energy yields, the chapter provides a foundation for predicting microbial activity and optimizing bioprocess design.