Sigil is a tabbed editor for the EPUB format commonly used in eBooks. The program includes basic formatting, customizable fonts and styles, hotkeys and spellcheck. You can generate a table of contents, embed files (audio, video, or other), edit/add metadata, rearrange or split pages, add vector (SVG) images and available "clips" for frequently entered text.

As the format is based on simple HTML, the program includes a WYSIWYG-style book vs. code view, the ability to modify cascading style sheets and additional functionality via plugins. The program can even shrink some eBooks by discarding unused media elements.

Cross-platform with clients for Linux, Mac and supports many languages. Sigil Portable is a wrapper version of the program and an outdated X-Sigil is also available.