This part explores the evolution of fed-batch fermentation from a basic nutrient replenishment method into a sophisticated metabolic control architecture. It explains how precisely managing the substrate feed rate allows engineers to dictate intracellular flux, prevent wasteful overflow metabolism, and protect the cell's respiratory capacity. The source categorizes various feeding strategies, ranging from predefined recipes to adaptive feedback systems like pH-stat and DO-stat control. Furthermore, it analyzes the physical constraints of high-cell-density operations, such as oxygen transfer limits and changes in broth rheology. Ultimately, the document presents fed-batch operation as a vital tool for balancing biological productivity with industrial scalability and economic efficiency.