Plant growth, development, reproduction and environmental stress responses are tightly regulated by a complex network of signalling pathways. Plant hormones - including salicylic acid, ethylene, jasmonic acid, auxins, gibberellins, cytokinins, abscisic acid and brassinosteroids - have long been considered the major signalling molecules during those processes. However, the discovery that many different ( secreted) peptides are involved in signalling has stimulated intensive research, and this special issue reflects the latest developments in this dynamic field.