Extracellular ATP (eATP) is an important signaling agent in plant growth and the defense response to environmental stimuli. eATP has dual functions in plant cell signaling, which largely depend on its concentration in the extracellular matrix (ECM). A lethal level of eATP (extremely low or high) causes cell death, whereas a moderate level benefits plant growth and development. Ecto-apyrases help controlling eATP content in the ECM and thus contribute to mediating the plant growth and defense response to environmental stress. We summarize the discovery, generation and removal of eATP; the receptors of eATP and eATP signaling in plants; and highlight the physiological function of eATP under stress. We also give a brief outlook of future research of eATP in plants.