Peptide signalling molecules create diverse modular signals in animal systems, but it is only relatively recently that an expanding array of peptide signalling groups has been identified in plants. Representatives occur in moss although most are in angiosperms (both monocot and dicot) including many agronomically important crops. Some groups show high diversity within a species, whereas other peptide signalling groups are small or represented by a single peptide or only found in a single family of plants. Plant peptide signals regulate meristem organogenesis and growth, modulate plant homeostasis and growth, and recognise damage or imminent danger from pathogen attack. The peptide signalling molecules are secreted into the apoplast where they are often further proteolytically processed before acting on receptors in nearby or adjacent cells with all the hallmarks of paracrine molecules. Where the receptors have been identified, they are receptor-like kinases that form oligomers upon peptide binding and relay messages via phosphorylation cascades. The use of nitrogen rich amino acids in the signalling peptides was analysed and nitrogen scores were obtained that are higher than the mean nitrogen score for the overall average of the Arabidopsis proteome. These findings are discussed in terms of nutritional availability and energy use.