Beginning with his arrest for possession of marijuana December 24, 1965, Dr. Timothy Leary became embroiled in a very public series of court cases that sought initially to utilize the Free Exercise Clause of the United States Constitution to challenge established United States drug laws regarding marijuana. Though Leary’s attempts at using the Free Exercise Clause were unsuccessful, his case was eventually heard by the United States Supreme Court, who, in 1969, found major elements of the 1937 Marihuana Tax Law unconstitutional and overturned Leary’s conviction. This chapter will trace evolution of Leary’s defense argument from one based on religious freedom to one based on due process and the Fifth Amendment protection against self-incrimination. Leary’s initial freedom of religion defense was based on the success of the Native American Church’s 1964 California State Supreme Court ruling that protected that group’s sacramental use of peyote within its religious ceremonies. This chapter will also describe how the success of Native American Church case led Leary to popularize and speak out in favor of small groups of psychedelic users establishing their own “official” religions. Leary followed his own advice by creating the League for Spiritual Discovery in 1966 and was seen as a religious leader by the founder of the Neo-American Church (another psychedelic quasi-religion) as well. Seeing themselves as a persecuted people under legal attack for their spiritual and experimental practices, both the Neo-American Church and the League for Spiritual Discovery sought to emulate the Native American Church and establish legal protections for the use of illegal substances.