Azole-based cyclic peptides found in ascidians ("sea squirts") of the genus Lissoclinum have a high propensity to chelate metal ions. This Highlight summarises the current evidence for marine cyclic peptide-metal congruence, and the structural and stereochemical features in cyclic peptides which seem necessary to facilitate metal complexation. The biological relevance of the metal ions in these associations, including their possible role in the assembly of cyclic peptides in the marine milieu, is also briefly considered. Finally, the synthesis of natural, and some novel non-natural, azole-based cyclic peptides from the cyclooligomerisation and assembly of azole-based amino acids, including in the presence of metal ions, is presented.