Post-translational modifications (PTMs) add tremendous complexity to cellular proteomes. The large variety of these modifications and their concurrent appearance in proteins dramatically increase the proteome size from mere thousands to the order of millions of possible protein forms. Emerging evidence from plants indicates that PTMs intimately regulate numerous developmental pro-grammes and responses to the environment. Reviews and research papers in this issue highlight our advanced knowledge of mechanisms that underpin several key PTMs and how these generate phenotypic traits that may be exploited to boost crop production.