Cells need energy to grow and reproduce. DNA replication and cell division require energy to acquire and incorporate nonliving substances into new cells. A major way cells gain useful energy is through catabolism, the free energy–releasing breakdown of large molecules into smaller molecules. This chapter covers fundamentals of energy transformations from the abstract physical level to the concrete cellular level. General concepts such as ATP as an energy carrier and specific catabolic pathways are explored. For microbes, the variety of such catabolic reactions is nearly limitless; virtually any kind of molecule in our biosphere—from hydrogen gas to chlorinated pollutants—can yield energy for some kind of microbe.