Adenosine 5'-triphosphate (ATP), a ubiquitously dispersed biomolecule, is not only a major source of biochemical energy for living cells, but also acts as a critical signaling molecule through inter-cellular communication. Recent studies have clearly shown that extracellular ATP is involved in various physiological processes in plants, including root growth, stomata movement, pollen tube development, gravitropism, and abiotic/biotic stress responses. The first plant purinergic receptor for extracellular ATP, DORN1 (the founding member of the P2K family of purinergic receptors), was identified in Arabidopsis thaliana by a forward genetic screen. DORN1 consists of an extracellular lectin domain, transmembrane domain, and serine/threonine kinase, intracellular domain. The predicted structure of the DORN1 extracellular domain revealed putative key ATP binding residues but an apparent lack of sugar binding. In this chapter, we summarize recent studies on the molecular mechanism of plant recognition of extracellular ATP with specific reference to the role of DORN1.