Crossref is a not-for-profit organization with over 10,000 member organizations that registers, links, and distributes metadata for scholarly research outputs. It maintains a database of over 99 million content items, including journals, books, conference proceedings, and other materials. Crossref provides services to help publishers, reference managers, and other groups discover, cite, and assess scholarly research. Members join Crossref to help discover and track where their content is located online and to participate in collaborative initiatives around metadata and technology.