AUCTeX is an extensible package for writing and formatting TeX files in Emacs and XEmacs. AUCTeX provides syntax highlighting, smart indentation and formatting, previews of mathematics and other elements directly in the editing buffer, smart folding of syntactical elements, macro and environment completion. It also supports the self-documenting .dtx format from the LaTeX project and, to a limited extent, ConTeXt and plain TeX. AUCTeX is distributed under the GNU General Public License.