Transpersonal psychology is a school of psychology that developed from the Humanistic Psychology movement in the mid 1960s, being founded by Abraham Maslow and Stan Grof. Whereas Humanistic Psychology was called the "Third Force" (after psychoanalysis and behavioural psychology), Transpersonal psychology or the transpersonal movement became the fourth force. It was part of the whole radical wave of consciousness revolution that occured in the 1960s through to early 70s. I remember when I started University at La Trobe in 1979, I was imemdiately drawn to Transpersobnal Psychology and the back issues of their journal in the university library. Important writers that I read at this time included Aurthur Deikman, James Welwood, Kenneth Ring, Ken Wilber, and others.
The Wikipedia page mentions that past presidents of the Association for Transpersonal Psychology (ATP), founded in 1972, "include Alyce Green, James Fadiman, Frances Vaughan, Arthur Hastings, Daniel Goleman, Robert Frager, Ronald Jue, Jeanne Achterberg and Dwight Judy. In the 1980s and 90s the field developed through the works of such authors as Jean Houston, Stanislav Grof, Ken Wilber, Michael Washburn, Frances Vaughan, Roger Walsh, Stanley Krippner, Michael Murphy, Charles Tart, David Lukoff, Vasily Nalimov and Stuart Sovatsky. " This gives a good list of names of movers and shakers in the Transpersonal Psychology field. Wilber himself jhas since dissociated hiomself from the movement and set up his own Integral movement.
In contrast to Humanistic psychology, Transpersonal Psychology /Transpersonal movement was strongly influenced by the Eastern spiritual teachings such as Buddhism, Advaita Vedanta, Sufism (but only via Idries Shah), and Taoism. Western influences include Carlos Castennada (if I remember rightly) and the human potential movement, meditation studies (like wiring up the brains of Zen meditators), Jung, and humanistic psychology.
According to Bahman Shirazi
"The cardinal contribution of transpersonal psychology has been the inclusion of the spiritual dimension of human life into the larger picture of psychological inquiry primarily through importing and borrowing from mystical and spiritual traditions, both Eastern and Western as well as indigenous traditions around the world. Despite the emphasis on higher values and human potentials, and self-actualization process by humanistic psychologists, issues of ego-transcendence and higher states of consciousness did not occupy as prominent a place in humanistic psychology as it has with transpersonal psychologists."
Transpersonal Psychology and the Transpersonal movement was hugely influential on the New Paradigm movement of the early 1980s. It seems less important now, not because it has waned in influence, but because other \movements, such as the Global Mind Shift, Conscious Evolutionists, and Integral Movement have developed since. One reason may be its lack of unifying center. While individuals liek Stan Grof and Ken Wilber went on to be extremely influential, Transpersonal Psychology as a whole never attained a unifying philosophical and spiritual orientation.
The Association for Transpersonal Psychology home page
Transpersonal Psychology - Wikipedia