Nucleotides and nucleosides are primitive biological molecules that were utilized early in evolution both as intracellular energy sources and as extracellular signalling molecules. ATP was first identified as a neurotransmitter and later as a co-transmitter with all the established neurotransmitters in both peripheral and central nervous systems. Four subtypes of P1 (adenosine) receptors, 7 subtypes of P2X ion channel receptors and 8 subtypes of P2Y G protein-coupled receptors have currently been identified. Since P2 receptors were first cloned in the early 1990’s, there is clear evidence for the widespread distribution of both P1 and P2 receptor subtypes in neuronal and non-neuronal cells, including glial, immune, bone, muscle, endothelial, epithelial and endocrine cells.