Peptide signaling plays a major role in various aspects of plant growth and development, as has been shown in recent biochemical, genetic and bioinformatic studies. There are over a dozen secreted peptides recognized in plants known to regulate cellular functions. To become functional, these secreted peptide signals often undergo post-translational modifications, such as tyrosine sulfation, proline hydroxylation, and hydroxyproline arabinosylation, and successive proteolytic processing. These types of ‘small post-translationally modified peptide signals’ are one of the major groups of peptide signals found in plants. In parallel with the discovery of peptide signals, specific receptors for such peptide signals were identified as being membrane-localized leucine-rich repeat receptor kinases. This short review highlights the recent progress in research on small post-translationally modified peptide signals, including our own research.