This article aims to investigate the Buddhist origins of Mindfulness-based Stress Reduction (MBSR) currently prevalent in the West, and the mechanisms through which MBSR contributes to mental and physical health of human beings. It first discusses the historical origins of MBSR. Then it explores in detail the two major elements which constitute mindfulness practices as taught in MBSR, that is, the teaching of "insight meditation" (vipassanā) in Theravāda Buddhism and the teaching of "Zen/Chan meditation" in East Asian Buddhism. Toward the end of this article, it examines, from the perspective of Thearvādin Abhidhamma, the mechanisms through which MBSR improve effectively human health and heal mental and physical disorders.