The jnana-kanda of the Veda — the knowledge portion. Adi Shankaracharya commented on these ten as the prasthana-trayi root. Total: 1507 mantras across 10 principal Upanishads. The Muktika Upanishad lists 108 Upanishads in total — these ten are the most important.
The Four Mahavakyas — one per Veda
प्रज्ञानं ब्रह्म (prajñānaṃ brahma) — "Consciousness is Brahman." Aitareya Upanishad 3.1.3 · Rig Veda
अहं ब्रह्मास्मि (ahaṃ brahmāsmi) — "I am Brahman." Brihadaranyaka Upanishad 1.4.10 · Shukla Yajur Veda
तत्त्वमसि (tat tvam asi) — "That thou art." Chandogya Upanishad 6.8.7 · Sama Veda
अयमात्मा ब्रह्म (ayam ātmā brahma) — "This Self is Brahman." Mandukya Upanishad 1.2 · Atharva Veda
The Ten Principal Upanishads
1. Isha Upanishad (ईशावास्य उपनिषद्)
Yajur (Shukla) Veda · 18 mantras · The Lord pervades all; renunciation through enjoyment
The shortest principal Upanishad. Opens with the famous "Isha vasyam idam sarvam" — the entire universe is enveloped by the Lord. Teaches simultaneous engagement (karma) + renunciation (tyaga). Reconciles action and knowledge in a single 18-mantra arc.
1.1 — īśāvāsyam-idaṃ sarvaṃ yat kiñca jagatyāṃ jagat | tena tyaktena bhuñjīthā mā gṛdhaḥ kasya svid dhanam ||
All this — whatsoever moves in this moving world — is to be enveloped by the Lord. Renouncing this, enjoy. Do not covet anyone's wealth.
1.5 — tad ejati tan naijati tad dūre tad vad antike | tad antarasya sarvasya tad u sarvasyāsya bāhyataḥ ||
That moves, that moves not. That is far, that is near. That is within all this, and that is also outside all this.
2. Kena Upanishad (केन उपनिषद्)
Sama Veda · 35 mantras · By whom (kena) is the mind moved? The unknowable Brahman
Opens with the question kena — "by whom?". Identifies Brahman as the unmoved mover behind every cognition — that which is the eye of the eye, the ear of the ear. Includes the famous Yaksha-katha — Indra fails to recognise Brahman even when Brahman tests him in disguise.
1.4 — yad vācā'nabhyuditaṃ yena vāg abhyudyate | tad eva brahma tvaṃ viddhi nedaṃ yad idam upāsate ||
That which is not uttered by speech but by which speech itself is uttered — know That alone to be Brahman, and not this that people here worship.
2.3 — yasyāmataṃ tasya mataṃ mataṃ yasya na veda saḥ | avijñātaṃ vijānatāṃ vijñātam avijānatām ||
He by whom It is not known, knows It; he by whom It is known, knows It not. It is unknown to those who know; it is known to those who do not know.
3. Katha Upanishad (कठ उपनिषद्)
Yajur (Krishna) Veda · 119 mantras · Nachiketa's three boons; Yama teaches Atma-vidya
Nachiketa, sent by his father to Yama, asks three boons. The third — what happens after death? — Yama tries to deflect. Nachiketa refuses every bribe. Yama then teaches the Atma-vidya. Source of the famous "uttiṣṭha jāgrata prāpya varānnibodhata" — Arise, awake, having attained the boon-givers, realise the goal.
1.3.14 — uttiṣṭhata jāgrata prāpya varān nibodhata | kṣurasya dhārā niśitā duratyayā durgaṃ pathas tat kavayo vadanti ||
Arise! Awake! Approach the great ones (gurus) and learn. Sharp as a razor's edge is the path — the wise call it hard to tread.
1.2.20 — aṇor aṇīyān mahato mahīyān ātmā'sya jantor nihito guhāyām | tam akratuḥ paśyati vītaśoko dhātu-prasādān mahimānam ātmanaḥ ||
Smaller than the smallest, greater than the greatest — the Self is hidden in the cave of the heart of every being. He who is desireless and free of sorrow beholds, by the grace of the creator, the glory of the Self.
4. Prashna Upanishad (प्रश्न उपनिषद्)
Atharva Veda · 67 mantras · Six questions to Pippalada; the science of prana
Six rishis approach Pippalada with six fundamental questions about creation, sustenance, and dissolution. Pippalada's answers crystallise the Vedic teaching on prana, on the syllable Om, on the sixteen kalas of the Purusha. The six prashnas constitute six full chapters.
3.3 — ātmana eṣa prāṇo jāyate | yathaiṣā puruṣe chāyaitasminnetadātataṃ manokṛtenāyātyasmin śarīre ||
From the Self alone is this prana born. As the shadow of a person is cast upon that person, so this prana is spread out by the working of the mind in this body.
5. Mundaka Upanishad (मुण्डक उपनिषद्)
Atharva Veda · 65 mantras · Para-vidya + apara-vidya; the two birds
Distinguishes apara-vidya (lower knowledge — Vedas, vyakarana, jyotisha, the four added Vedangas) from para-vidya (higher knowledge — direct realisation of the Akshara Brahman). Famous "Satyam-eva jayate" mahavakya lives in 3.1.6. The two birds on the same tree (3.1.1) — one eats the fruit, the other watches.
3.1.1 — dvā suparṇā sayujā sakhāyā samānaṃ vṛkṣaṃ pariṣasvajāte | tayoranyaḥ pippalaṃ svādvattyanaśnan anyo abhicākaśīti ||
Two birds, inseparable companions, perch on the same tree. One eats the sweet fruit; the other, not eating, watches.
3.1.6 — satyam eva jayate nānṛtaṃ satyena panthā vitato devayānaḥ | yenākramantyṛṣayo hyāptakāmā yatra tat satyasya paramaṃ nidhānam ||
Truth alone triumphs, not falsehood. By truth is laid out the path leading to the gods — that path by which the sages tread, their desires fulfilled, where lies the highest treasure of Truth.
6. Mandukya Upanishad (माण्डूक्य उपनिषद्)
Atharva Veda · 12 mantras · The four states + the syllable Om
The most condensed Upanishad. 12 mantras cover the four states (jagrat-svapna-sushupti-turiya) corresponding to the four mātrās of Om (a-u-m-silence). The mahavakya "ayam ātmā brahma" is here. Gaudapada's Karika (~200 verses) expanded these 12 mantras into the foundation Advaita text.
1.2 — sarvaṃ hyetad brahma ayam ātmā brahma so'yam ātmā catuṣpāt |
All this is verily Brahman. This Self is Brahman. This Self has four parts (i.e. four states).
1.1 — om ity etad akṣaram idaṃ sarvaṃ tasyopavyākhyānaṃ bhūtaṃ bhavad bhaviṣyad iti sarvam oṅkāra eva |
Om — this syllable is all this. Its full explanation: whatever was, is, and shall be is all simply Om.
7. Aitareya Upanishad (ऐतरेय उपनिषद्)
Rig Veda · 33 mantras · Creation; the Self as the only doer
Found in the Rig Veda's Aitareya Aranyaka. Three short chapters describe creation (Atma alone existed; he created the worlds), the formation of the embryo, and the first mahavakya — "Prajnanam Brahma" (consciousness is Brahman). The seer Aitareya Mahidasa is credited.
1.1.1 — ātmā vā idam eka evāgra āsīn nānyat kiñcana miṣat | sa īkṣata lokān nu sṛjā iti ||
In the beginning the Atman alone existed — no other thing whatsoever was. He desired: "let me create the worlds".
3.1.3 — prajñānaṃ brahma
Consciousness is Brahman. — The Rig-Veda mahavakya.
8. Taittiriya Upanishad (तैत्तिरीय उपनिषद्)
Yajur (Krishna) Veda · 96 mantras · Five koshas; "satyaṃ vada · dharmaṃ cara"
Three vallis (chapters) — Shiksha-valli (pronunciation + the disciple's farewell speech "satyam vada"), Brahmananda-valli (the five sheaths — annamaya through anandamaya), and Bhrigu-valli (Bhrigu's tapasya under his father Varuna, discovering Brahman as annam → prana → manas → vijnana → ananda).
1.11.1 — satyaṃ vada | dharmaṃ cara | svādhyāyān mā pramadaḥ | ācāryāya priyaṃ dhanam āhṛtya prajātantuṃ mā vyavacchetsīḥ |
Speak the truth. Practise dharma. Do not neglect your study. Having brought your teacher his fitting present, do not cut off the thread of your descendants.
2.7 — raso vai saḥ | rasaṃ hy evāyaṃ labdhvā''nandī bhavati |
He, verily, is rasa (essence, sweetness). Having attained this rasa, one becomes blissful.
9. Chandogya Upanishad (छान्दोग्य उपनिषद्)
Sama Veda · 627 mantras · "Tat tvam asi" — Uddalaka to Shvetaketu
Largest Upanishad after Brihadaranyaka. Eight prapathakas. Sixth prapathaka contains Uddalaka Aruni's teaching to his son Shvetaketu — the famous nine-fold repetition of the mahavakya "tat tvam asi". Also the Sandilya-vidya (3.14), the Bhuma-vidya (7.23), and the Dahara-vidya (8.1).
6.8.7 — sa ya eṣo'ṇimaitad-ātmyam idaṃ sarvaṃ tat satyaṃ sa ātmā tat tvam asi śvetaketo |
That which is the subtle essence — in It all that exists has its self. That is the Truth. That is the Self. That thou art, Shvetaketu.
3.14.1 — sarvaṃ khalv idaṃ brahma tajjalān iti śānta upāsīta |
All this verily is Brahman. From Him it arises, in Him it dissolves, in Him it abides. With peace, let one worship Him.
10. Brihadaranyaka Upanishad (बृहदारण्यक उपनिषद्)
Yajur (Shukla) Veda · 435 mantras · "Aham brahmasmi"; Yajnavalkya-Maitreyi dialogue
The largest Upanishad. Six adhyayas, 47 brahmanas. Famous for: the Yajnavalkya-Janaka dialogues, the Yajnavalkya-Maitreyi dialogue (the husband departs for sannyasa; his wife asks not for wealth but for what makes him truly immortal), the Madhu-vidya, and the great prayer "asato ma sad gamaya" (lead me from the unreal to the real).
1.3.28 — asato mā sad gamaya | tamaso mā jyotir gamaya | mṛtyor mā'mṛtaṃ gamaya |
From the unreal lead me to the real. From darkness lead me to light. From death lead me to immortality.
1.4.10 — ahaṃ brahmāsmi
I am Brahman. — The Shukla Yajur Veda mahavakya.
2.4.5 — na vā are patyuḥ kāmāya patiḥ priyo bhavaty ātmanas tu kāmāya patiḥ priyo bhavati |
Not for the husband's sake, my dear, is the husband loved; rather for the sake of the Self the husband is loved. (Yajnavalkya to Maitreyi)
Prasthana-trayi
The three pillars of Vedanta — the 10 Upanishads (revealed canon), the Bhagavad Gita (smriti pillar), and the Brahma Sutras of Badarayana (logical pillar). Every Vedanta acharya — Shankara, Ramanuja, Madhva — wrote bhashyas on all three.