Windows Live Hotmail, formerly known as MSN Hotmail and commonly referred to simply as Hotmail, is a free web-based email (webmail) service operated by Microsoft as part of its Windows Live group. It was founded by Sabeer Bhatia and Jack Smith (Jack Smith (Hotmail)). Launching in July 1996 as "HoTMaiL" and funded by the venture capital firm Draper Fisher Jurvetson, it was one of the first web-based e-mail services. Its original name and capitalization refers to HTML, the encoding language used by the World Wide Web. It was also one of the first free e-mail providers. It was subsequently acquired by Microsoft in 1997 for an estimated $400 million, and shortly after it was rebranded as "MSN Hotmail". The current version, "Windows Live Hotmail", was officially announced in 2005 and released worldwide in 2007.

Windows Live Hotmail features 5GB of storage that expands as necessary, security measures for which patents have been filed, Ajax technology (Ajax (programming)), and integration with Windows Live Messenger, Spaces (Windows Live Spaces), Calendar (Windows Live Calendar), and Contacts (Windows Live Contacts). It has over 360 million users worldwide as of May 2010. It is available in 36 different languages.

The Hotmail development and operations groups are based in Mountain View, California. When Hotmail Corporation was an independent company, its headquarters were in Sunnyvale, California.

On May 18, 2010, Microsoft unveiled a new version of Hotmail which offers features such as 1-Click Filters, Active Views, Inbox Sweeping, and integration with Windows Live SkyDrive and Windows Live Office (a free version of Microsofts Office Web Apps suite). The new version is slated to be released on June 15, 2010 and rolled out to Hotmails 360 million users over the course of the coming months.