Heaven's Gate was an American new religious movement, often described as a cult. It was founded in 1974 and led by Bonnie Nettles (1927–1985) and Marshall Applewhite (1931–1997), known within the movement as Ti and Do, respectively. Nettles and Applewhite first met in 1972 and went on a journey of spiritual discovery, identifying themselves as the two witnesses of Revelation, attracting a following of several hundred people in the mid-1970s. In 1976, the group stopped recruiting and instituted a monastic lifestyle.

Scholars have described the theology of Heaven's Gate as a mixture of Christian millenarianism, New Age, and ufology, and as such it has been characterized as a UFO religion. The central belief of the group was that followers could transform themselves into immortal extraterrestrial beings by rejecting their human nature, and they would ascend to heaven, referred to as the "Next Level" or "The Evolutionary Level Above Human". The death of Nettles to cancer in 1985 challenged the group's views on ascension, where they originally believed that they would ascend to heaven while alive aboard a UFO, later coming to believe that the body was merely a "container" or "vehicle" for the soul, and that their consciousness would be transferred to new "Next Level bodies" upon death.

On March 26, 1997, deputies of the San Diego County Sheriff's Department discovered the bodies of the 39 active members of the group, including that of Applewhite, in a house in the San Diego suburb of Rancho Santa Fe. They had participated in a mass suicide, a coordinated series of ritual suicides, coinciding with the closest approach of Comet Hale–Bopp. Just before the mass suicide, the group's website was updated with the message: 'Hale–Bopp brings closure to Heaven's Gate ...our 22 years of classroom here on planet Earth is finally coming to conclusion—'graduation' from the Human Evolutionary Level. We are happily prepared to leave 'this world' and go with Ti's crew.'The name "Heaven's Gate" was only used for the final few years of the group's existence, and they had previously been known under the names Human Individual Metamorphosis and Total Overcomers Anonymous.