[Inner Traditions] One of the most important books written on the effects of LSD on the human psyche. • Its authoritative research has great relevance to the current debate on drug legalization. • Prolific authors Robert Masters and Jean Houston are pioneer figures in the field of transpersonal psychology and founders of the Human Potentials Movement. The Varieties of Psychedelic Experience was published in 1966, just as the first legal restrictions on the use of psychedelic substances were being enacted. Unfortunately, the authors' pioneering work on the effects of LSD on the human psyche, which was viewed by its participants as possibly heralding a revolution in the study of the mind, was among the casualties of this interdiction. As a result, the promising results to which their studies attested were never fully explored. Nevertheless, their 15 years of research represents a sober and authoritative appraisal of what remains one of the most controversial developments