The TEI Guidelines have been developed and are maintained by the Text Encoding Initiative Consortium (TEI) and are designed for people working with any kind of textual resource in digital form.

The guidelines present recommendations of suitable ways of representing the features of textual resources, which need to be identified explicitly in order to facilitate processing by computer programs. In particular, they specify a set of markers (or tags) which can be inserted in the electronic representation of the text, to mark the text structure and other features of interest. Many computer programs depend on the presence of such explicit markers for their functionality. Without the markers, a digitized text appears to be nothing but a sequence of undifferentiated bits. The success of the World Wide Web, for example, is partly because of its use of such markup to indicate features, such as headings and lists on individual pages, and to indicate links between pages. The process of inserting such explicit markers for implicit textual features is often called ‘markup’, or equivalently within this work ‘encoding’; the term ‘tagging’ is also used informally. We use the term encoding scheme or markup language to denote the complete set of rules associated with the use of markup in a given context. Besides, we use the term markup vocabulary for the specific set of markers or named distinctions employed by a given encoding scheme. Thus, this work describes both the TEI encoding scheme, and documents the TEI markup vocabulary.

The TEI encoding scheme is particularly useful in facilitating a loss-free interchange of data amongst individuals and research groups using different programs, computer systems, or application software. Since they contain an inventory of the features most often deployed for computer-based text processing, the guidelines are also useful as a starting point for those designing new systems and creating new materials, even where interchange of information is not a primary objective.