Metabarcoding of complex metazoan communities is increasingly being used to measurebiodiversity in terrestrial, freshwater, and marine ecosystems, revolutionizing our ability toobserve patterns and infer processes regarding the origin and conservation of biodiversity. Afundamentally important question is which genetic marker to amplify, and although themitochondrial cytochrome oxidase subunit I (COI) gene is one of the more widely usedmarkers in metabarcoding for the Metazoa, doubts have recently been raised about itssuitability. We argue that (i) the extensive coverage of reference-sequence databases for COI,(ii) the variation it presents, (iii) the comparative advantages for denoising protein codinggenes, and (iv) recent advances in DNA sequencing protocols argue in favour ofstandardising for the use of COI for metazoan community samples. We also highlight whereresearch efforts should focus to maximise the utility of metabarcoding.