This chapter introduces the fundamental role of stoichiometry and bacterial energetics in environmental biotechnology, with a focus on wastewater treatment processes. Mass balance equations are presented as essential tools for quantifying substrate utilization, electron transfer, and biomass formation. The discussion emphasizes how electron donors, electron acceptors, and nutrient requirements shape microbial metabolism and influence end products such as carbon dioxide, water, and new biomass. By linking oxidation–reduction reactions with energy conservation in microorganisms, the chapter establishes the basis for predicting yields, designing bioprocesses, and optimizing biological treatment systems.