This chapter focuses on anabolism, the biosynthetic arm of microbial metabolism, where energy derived from catabolism is utilized to build complex cellular components. It describes how ATP and reducing equivalents drive the synthesis of proteins, nucleic acids, lipids, and polysaccharides, all essential for biomass growth. The importance of anabolism in wastewater treatment is highlighted, as new cell formation underlies pollutant removal and sludge production. Stoichiometric approaches are introduced to quantify the partitioning of electrons between energy generation and cell synthesis, linking catabolic and anabolic reactions. The chapter also discusses simplified models, such as the generic cell formula (C5H7O2N), to predict biomass yields and metabolic balances in engineered systems.