An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. Facultative anaerobes grow and survive in an oxygenated environment and so do aerotolerant anaerobes.

A good example would be the oxidation of glucose (a monosaccharide) in aerobic respiration.

Notice that oxygen is used during the oxidation of glucose and water is produced.

This equation is a summary of what actually happens in three series of biochemical reactions: glycolysis, the Krebs cycle, and oxidative phosphorylation.

Almost all animals, most fungi, and several bacteria are obligate aerobes, which require oxygen. Most anaerobic organisms are bacteria.^{[clarification needed]} Being an obligate aerobe, although advantageous from the energetical point of view, also means obligatory exposure to high levels of oxidative stress.

Yeast is an example of a facultative anaerobe, which can develop in the presence of oxygen but does not require it. Individual human cells are also facultative anaerobes: they switch to lactic acid fermentation if oxygen is not available. However, for the whole organism this cannot be sustained for long, and humans are therefore obligate aerobes.