Aerobic respiration, a process that uses oxygen, and anaerobic respiration, a process that doesn't use oxygen, are two forms of cellular respiration. Although some cells may engage in just one type of respiration, most cells use both types, depending on an organism's needs. Cellular respiration also occurs outside of macro-organisms, as chemical processes — for example, in fermentation. In general, respiration is used to eliminate waste products and generate energy.