Aoyama Gakuin University This document defines best practices for defining new top-level media types. It also introduces a registry for top-level media types, and contains a short history of top-level media types. It updates RFC 6838. [RFC Editor, please remove this paragraph.] Comments and discussion about this document should be directed to media-types@ietf.org, the mailing list of the Media Type Maintenance (mediaman) WG. Alternatively, issues can be raised on GitHub at https://github.com/ ietf-wg-mediaman/toplevel.