This is an indepth study and translation of the Isopanisad. The Isopanisad is one of the earliest of the sacred texts of Hinduism called the Upanisads. The Upanisads ushered in a new period (600-200 BCE) in the history of Vedic/Hindu thought, religion, and culture. In the period before (1200-700 BCE) people focused on ritual action, specifically sacrifice, as the best means of achieving the good things of life, both in this world and in the next. The Upanisads, coming at the end of this period, questioned the assumptions behind the earlier practices and beliefs and settled on the pursuit of knowledge as the best means of gaining the highest good, which they envisioned not as goods or life in heaven, but as absolute freedom from suffering and fear. This translation includes word-by-word analysis of this eighteen-mantra Upanisad which presents the essential teachings of Vedanta (the various schools of which attempt to derive unified systems of thought out of the Upanisads). Two native commentaries, that of Sankara (7th cent. CE) and of Madhva (13th cent. CE), are included as well as an introduction, notes, discussions, study questions, a bibliography for further reading, a historical and text-critical essay on the Isopanisad by Croatian scholar, Mislav Jezic, and an introduction to the Sanskrit language of the Upanisad. The full Sanskrit texts of the commentaries of Sankara and Madhva are also included.