A MOBI file is an eBook saved in the MOBI format, a format originally used by the Mobipocket Reader but now supported by several different readers. It contains an eBook and may incorporate DRM copyright protection to prevent copying or unauthorized viewing. MOBI files are supported by various eReaders, tablets, PDAs, and desktop computer applications.

MOBI file open in Calibre 3 Amazon's new Kindle Format 8 (KF8) is based on the MOBI format. KF8 supports many new formatting and layout options, including HTML5 and CSS3 content and embedded fonts. KF8-formatted eBooks are saved and distributed as ".mobi" files or as .AZW3 files, depending on the device that downloads the eBook. eBook authors can compile KF8 books using Amazon's KingleGen software. Before distribution, authors can preview KF8 eBooks on a Mac or PC with Amazon's Kindle Previewer software. Amazon now owns the MOBI format. Amazon's eBook formats, including .AZW, .AZW1, and AZW3, use the MOBI format but incorporate a proprietary DRM scheme. NOTE: Mobipocket eBooks also use the .PRC extension.