The Text Encoding Initiative (TEI) Guidelines make recommendations about suitable ways of representing those features of textual resources which need to be identified explicitly in order to facilitate processing by computer programs. They specify a set of XML tags in order to mark the textual metadata, text structure, relationship between images and transcriptions and other features of interest. They therefore primarily define a data format, but the TEI Header in particular includes a native set of metadata and may include metadata from other schemas.

In their decades of community driven development they have developed into a de-facto standard in the production of textual data in the humanities. Since the release of version P5, 2 to 4 new revisions have been released each year.