Nitric oxide (NO) has recently joined the select circle of the ubiquitous molecules of plant signalling networks. Indeed, the last decade has produced a tremendous amount of data that evidence the diversity of physiological situations in which NO is involved in plants and the complexity of NO biology. These data also underline our difficulties in providing simple answers to the cardinal questions of where NO comes from and how the NO message is converted into a physiological response. The identification of NO primary targets and NO-regulated genes provides new opportunities to connect NO biochemistry and NO biology. This review summarises our current understanding of NO signalling, from the generation of the NO message to its execution into a cellular response. The review particularly considers whether and how NO may be responsible for specific signalling in different physiological processes.