The technology of the ePub format
ePub is an open, royalty free standard for e-books, maintained by the World Wide Web Consortium.
A file with the .epub extension stores a book, or any other structured publication, as a package of web content. EPUB is short for Electronic PUBlication. It can be read on any device with suitable software, which is the point: a publisher produces one file and it works everywhere.
What is inside the file
An ePub file is a ZIP archive. Rename one to .zip, open it, and you can see its parts.
XHTML documents. The text itself, one file per chapter in most books. XHTML is a stricter form of HTML, and the strictness is deliberate: a reading app can render it without guessing at broken markup.
Packaging files. The package document, with an .opf extension, lists every file in the book and the order they are read in. Older books also carry an .ncx file for the table of contents. EPUB 3 replaced that with a navigation document written in XHTML, so .ncx now appears mainly for backward compatibility.
The container. A folder named META-INF holds the file that tells a reader where the package document is. If the book carries digital rights management, the encryption details live here too. Without DRM, the container is a few standard lines.
How the specification has changed
The first version was published in 2001, as XHTML with embedded fonts. In 2003 the International Digital Publishing Forum released its second edition, Open eBook.
EPUB 3.0 was approved in 2011 and brought the format onto modern web standards: HTML5, CSS3, embedded audio and video, and scripting. Version 3.2 followed in 2019.
The governance then changed. The IDPF merged into the W3C in 2017, and EPUB is now maintained there as a web standard. EPUB 3.3 became a W3C Recommendation on 25 May 2023. It is backward compatible with 3.2, so a working production pipeline did not need changing, and for the first time the accessibility specification became part of the standard itself rather than a separate document.
DRM is optional, and it is not part of the format
An ePub file can carry digital rights management as an extra layer, which restricts what a reader may do with the book and which reading apps can open it.
This is worth being precise about. DRM is not part of the ePub specification. A book bought from one shop and a DRM free book from another can both be valid ePub files, and only one of them will open anywhere.
Reviewed September 2026. The specification and governance details above were updated: EPUB is maintained by the W3C rather than the IDPF, and 3.3 has superseded the 3.2 version this article originally ended on.