.md File Extension
The .md file extension names a plain-text file whose contents are written in Markdown, a lightweight markup language for adding formatting elements to ordinary text documents[^c1]. Markdown and its file extension were created in 2004 by John Gruber and Aaron Swartz[^c2], and a document written in the syntax is stored as a plain-text file bearing the .md or .markdown suffix[^c3]. Because the file contains only text, it can be opened in any editor, versioned with ordinary tools, and converted to HTML, PDF, or other formats.
Markdown spread from its origins as a text-to-HTML tool for web writers into a general-purpose notation used across GitHub, Reddit, Stack Exchange, and many other platforms, and it is now fair to describe it as adopted by the software community at large[^c4]. It is the default format for README files and technical documentation, the basis of static site generators, note-taking applications, and blogging workflows. The registered media type for the format is text/markdown, and its associated file extensions are .md and .markdown[^c5].
Since the 2020s the extension has acquired a further role as a document format optimised for the AI era[^c7]. Markdown files carry the memory, prompts, and control rules that AI agents are given[^c9], and the format is favoured by language models because it is plain text whose structural signals are local and inexpensive to process. Major platforms have responded by supporting it natively: Google Drive and Google Docs added full support for opening, editing, and collaborating on .md files[^c8], Microsoft made Markdown a first-class format in OneDrive and SharePoint so that Markdown files behave like any other file in the Microsoft 365 workflow[^c10], and LibreOffice 26.2 added Markdown import and export to the desktop office suite[^c13]. The format has also acquired shared conventions for machine use: the AGENTS.md instruction file, contributed by OpenAI to the Agentic AI Foundation in December 2025, was reported to be used by more than 60,000 open-source projects and more than 30 compatible tools[^c11]. AGENTS.md is one of three complementary Markdown instruction layers — alongside SKILL.md for reusable agent capabilities and Google Labs' DESIGN.md for visual design systems[^c14] — and a census of GitHub's most-starred repositories found an AGENTS.md in 35 per cent of them[^c15]. The extension now serves as a working format for machine-to-machine exchange as well as a publishing format for people.
A defining feature of the .md extension is that it identifies a format without a formally fixed specification: there is no single standard for the original Markdown, and its ambiguities are handled inconsistently by different implementations[^c6]. That gap gave rise to a family of flavors — CommonMark, GitHub Flavored Markdown, Markdown Extra, and MultiMarkdown among them — and to a long-running debate about whether the language should be standardised at all. Even CommonMark, the closest thing to a standard, remains a 0.x specification, and new proposals such as Markdown++ continue to extend the core while keeping every file valid as ordinary Markdown[^c12]. The extension is studied both as a file format and as the ecosystem it represents, from the internal structure of a .md file and the syntax of the language, through the tools that write, render, and convert it, to its history and the controversies that surround it.