Endogenously produced nitric oxide (NO) has numerous physiological roles. It is generated by three enzymes: eNOS, nNOS and iNOS. NO produced in endothelia by eNOS regulates blood vessel dilation by targeting vascular smooth muscle cells; NO produced in neurons by nNOS regulates synaptic signalling and plasticity in the brain; and NO produced in large quantities by iNOS in cells of the innate immune system can kill fungal and bacterial pathogens and inhibits viral replication. In the next three issues of JCS, contributors to Cell Science at a Glance survey these three scenarios. On p. 2427 of this issue, William Sessa tackles eNOS.