P2X receptors for ATP comprise a distinct family of ligand gated ion channels with a range of properties. They have been shown to be involved in a variety of physiological processes including blood clotting, sensory perception, pain sensation, bone formation as well as inflammation and may provide a number of novel drug targets. In addition to the orthosteric site for ATP binding it has been suggested that there may be additional allosteric sites that regulate agonist action at the receptor. There is currently no crystal structure available for P2X receptors and the lack of sequence similarity to other ATP binding proteins has meant that a mutagenesis-based approach has been used primarily to investigate receptor structure-function. This review aims to provide an overview of recent work that gives an insight into residues involved in ATP action and allosteric regulation.