Persistent identifiers (PIDs) uniquely identify researchers, organizations, and research outputs. PIDs allow these entities to be connected and provide stable references for citations. They offer advantages like clear and stable identities that allow humans and machines to accurately refer to data over many years. PIDs are the "plumbing" of open research as they enable findings and contributions to be traced, reused, and properly attributed. Funders are increasingly mandating the use of PIDs like ORCID to support open access policies and more transparent research evaluation. PIDs thus help enable open research and responsible, metrics-based evaluation of that research.