This document defines the Internationalized Resource Identifier (IRI) protocol element, as an extension of the Uniform Resource Identifier (URI). An IRI is a sequence of characters from the Universal Character Set (Unicode/ISO 10646). Grammar and processing rules are given for IRIs and related syntactic forms. Defining IRI as a new protocol element (rather than updating or extending the definition of URI) allows independent orderly transitions: protocols and languages that use URIs must explicitly choose to allow IRIs. Guidelines are provided for the use and deployment of IRIs and related protocol elements when revising protocols, formats, and software components that currently deal only with URIs. This document is part of a set of documents intended to replace RFC 3987.