Nitric oxide (NO) is a small simple molecule but one which is instrumental in cell signalling in a range of organisms including plants. It is a gas, often found in the form of a radical. It is produced in cells in an orchestrated manner, usually by dedicated enzymes and often following stress. It leads to numerous responses. In plants it is involved in seed germination, seedling development, stomatal responses, senescence and protection against pathogens, so is an instrumental signalling molecule used throughout the life of the plant. Responses to NO may involve the generation of other signalling molecules such as cGMP, or may involve the covalent modification of proteins in a process known as S-nitrosation. However, it should be noted that NO signalling will not be isolated in the cell and NO will impinge on other signalling pathways, such as those involving reactive oxygen species. Therefore NO should be considered as part of a suite of signalling components which enable plants to thrive and survive.