Shankara wrote commentaries on ten principal Upanishads, providing the interpretive framework through which these ancient mystical texts are understood. Each commentary demonstrates different facets of Advaita. Isha Upanishad (18 verses) balances action and knowledge: "He who sees all beings in the Self and the Self in all beings, hates none." Kena Upanishad asks "By whom?" (kena)—by what power do mind, senses, breath function? Answer: Brahman is the power behind powers. Katha Upanishad presents young Nachiketa's dialogue with Yama (Death) on immortality, containing famous chariot metaphor: body is chariot, intellect the driver, mind the reins, senses the horses, sense objects the roads. Prashna Upanishad addresses six questions about ultimate reality. Mundaka Upanishad distinguishes Para Vidya (higher knowledge of Brahman) from Apara Vidya (lower knowledge including Vedic rituals)—even Vedas are preparatory to direct realization. Mandukya Upanishad analyzes OM and four consciousness states (waking, dream, deep sleep, Turiya) in twelve dense verses—one of Advaita's most important texts. Aitareya Upanishad explores creation and proclaims "Prajnanam Brahma." Taittiriya Upanishad presents five sheaths (koshas) and defines Brahman as "that from which beings are born, by which they live, into which they return." Chandogya Upanishad contains multiple teachings including the famous section where Uddalaka teaches his son Svetaketu "Tat Tvam Asi" through nine analogies (salt in water, rivers merging in ocean, etc.). Brihadaranyaka Upanishad, the longest, contains Yajnavalkya's profound teachings: "The Self is not this, not this" (neti neti), "You cannot see the seer of seeing," descriptions of creation, and final teaching to his wife Maitreyi on immortality. Shankara's commentaries provide word-by-word analysis, resolve apparent contradictions, refute rival interpretations, and extract systematic Advaitic philosophy from poetic, often enigmatic original verses. They remain the standard interpretive tradition for these texts.