Crossref is a nonprofit organization that makes scholarly research outputs easy to find, cite, link and assess. It maintains metadata for over 95 million scholarly items, assigns DOIs as persistent identifiers, and offers services to register, link and distribute metadata. Crossref tools allow members to deposit and update metadata via XML files or a web form, and various reports inform members about DOI resolution and errors. Support is available through an online support center and by contacting Crossref technical support.