Almost all cellular activities, including movement, membrane transport, and the synthesis of cell constituents, require energy. Consequently, the generation and utilization of metabolic energy is fundamental to all of cell biology. All cells use ATP as their source of metabolic energy, and the mechanisms that cells use for the generation of ATP, either from the breakdown of organic molecules or from photosynthesis, are discussed in this chapter. In addition, this chapter presents an overview of the network of chemical reactions that constitute the metabolism of the cell and are responsible for the synthesis of major cell constituents, including carbohydrates, lipids, proteins, and nucleic acids.