AUM emerged out of Asahara's dissatisfaction with traditional Japanese Buddhism. Having found Tibetan (Tibetan Buddhism) and Theravada Buddhist teachings more impressive than the dominant forms of Japanese Buddhism, Asahara attempted to create a Buddhism that emphasized non-Japanese themes. He created a spiritual path whose goal was the attainment of enlightenment in this life. It incorporated a variety of techniques, from Yoga and meditation to psychic-development exercises, to assist the followers' growth toward enlightenment. Progress was marked by three levels of initiation, and after attaining the third level members could be admitted into the group's monastic community. Followers, who included many well-educated people, were taught to give total devotion to their guru. Asahara also became interested in prophecy, studied the Christian Book of Revelation (Revelation to John), and in the early 1990s predicted an array of disasters for Japan, including World War III. Because Asahara expected AUM to replace the Japanese government in the chaos following the war, its organization mirrored that of the government.

At the time of the gassing incident, March 20, 1995, AUM claimed some 50,000 members, the majority of whom lived in Russia. Following the arrest of Asahara and several hundred members of the leadership and rank and file and the conviction of scores of those arrested for both the subway gassing and numerous other violent acts, the group was decimated and the Japanese government seized its property. Although a contingent of the group remained in Japan, foreign affiliates dissolved in the wake of the gassing. In December 1999 AUM's new leaders apologized for the crimes committed by members of the group and pledged to compensate victims of the 1995 gassing. The next month they admitted Asahara's role in a series of crimes, including the gassing, distanced themselves from his spiritual leadership, set up a program to pay compensation to the victims' families, and changed the organization's name to Aleph. In the early 2000s the group had more than 1,500 members.

The trials of AUM members implicated in the gassing incident continued into the early 21st century, with more than 10 people receiving death sentences. In 2004, after an eight-year trial, Asahara was convicted of masterminding the attack and was also sentenced to death.