The transformation of energy, its storage, and its use are central to the economy of the cell. Energy may be defined as the ability to do work, a concept that is as applicable to cells as to automobile engines and electric power plants. The energy stored within chemical bonds can be harnessed to support chemical work and the physical movements of cells. In this section, we review how energy influences the extents of chemical reactions (chemical thermodynamics) and the rates of chemical reactions (chemical kinetics).