Though there are more than 200 Upanishads, there are 108 traditional Upanishads recognized as more significant. Of these, 10 are the principal Upanishads: Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya and Brhadaranyaka. Shankara wrote commentaries on all these Upanishads as well as onShvetashvatara, Kausitaki and Mahanarayana. These, together with the Maitri, constitute the 14 principal Upanishads. The authors of the Upanishads are not known. They belong to the Shruti literature, and are the utterances of sages who spoke from the fullness of their illumed experiences. Their aim is to give us the knowledge as a means of spiritual freedom. Some of the Upanishads are associated with renowned sages such as Aruni, Yajnavalkya, Balaki, Shvetaketu and Shandilya. They are thought to be exponents of the doctrines attributed to them. Later philosophers used the Upanishads to form their own opinions and create their own systems. Each Veda is divided into four parts, namely Samhita, Brahmana, Aranyaka and Upanishad. The Samhitas (hymns of the Vedas) were written by poets, the Brahmanas (the methodology of rituals) by the priests and the Aranyakas (forest-books, a prelude to Upanishads) and Upanishads by philosophers. 

Though there is no single, coherent theme in the Upanishads, there are certain fundamental doctrines that stand at the center of their teachings. The concept of one God, of whom all others are manifestations, is reflected on and takes root in the Upanishads. They also consistently reflect on the Self (Atman) as the Brahman. This is seen in the mahavakya, 'Tat tvam asi' (That art Thou). Truth is within us and theAtman and Brahman are one and the same, which can be discovered with introspection. Another mahavakya, 'Aham Brahmasmi', also denotes that Brahman is within you. The stages of 'inner ascent' are stressed in detail and sacrificial rites are not condoned. Ignorant, selfish way may result in transitory satisfaction but the eternal life is achieved only through introspection and understanding of the Self. This can only be obtained with disciplined life with fortitude and adequate moral preparation. Upanishads also make the distinction between Brahman without qualities (Nirguna) and with qualities (Saguna). Nirguana Brahman is incomprehensible, the pure Absolute. Saguna Brahman called Ishvara is the Brahman in the observable (phenomenal) universe and the Self in the individual selves. 

A summary of the principal Upanishads are discussed here:  

Isha Upanishad 

Derives its name form Ishvasya; it is one of the smallest of all the Upanishads and belongs to the Yajur Veda. It touches on many important topics including the paths of knowledge and paths of action. The former follows the Atman in its transcendent form and the latter in its immanent form. Neither of these two alone, without the other is sufficient for the true wisdom. It also deals with the paradoxical nature of the Self (far but not near, inside and outside at the same time etc.). A sage, who sees all beings in his Self and his Self in all beings, has attained the wisdom and has lost all fear, sorrow and delusion. It is interested not so much in the Absolute in itself (Parabrhaman), as in the Absolute in relationship to the world (Parameshvara). It ends with a plea to Sun, the God of Light and to Agni (fire) as well as the Creator to help the Atman understand and follow the paths of proper knowledge and action. 

Kena Upanishad 

The name Kena indicates the question 'By whom?' and belongs to the Sama Veda. The Upanishad explains the real power behind the functions of the phenomenal universe and the workings of the man. It asks and answers the question as to who the Atman is. The knowledge of the Absolute (Nirguna Brahman ' para vidya), leads to immediate liberation and emancipation (sadyo mukti). Knowledge of Ishvara (Saguna Brahman ' apara vidya) paves the way to a gradual acquisition of knowledge, eventually leads to liberation (krama-mukti). To indicate the paradoxical nature of Brahman, Kena Upanishad contains this famous quote, 'It is not understood by those who say they understand It. It is understood by those who say they understand It not'. 

Katha Upanishad 

Part of the black Yajur Veda, Katha Upanishad is perhaps the most philosophical of all the Upanishads. It contains the famous dialogue between Nachiketa and Yama, the lord of departed spirits. Nachiketa's father, in a rage, inadvertently promises his son to Yama as sacrifice, and to keep his promise, promptly sends him to the doors of Yama's abode. Finding Yama absent, Nachiketa is kept waiting for three days at the doors, without food or water. Yama, on his return, apologizes to Nachiketa as no guest is to be treated in such a manner in his household. He offers three boons to Nachiketa. Return to his father in earth and the meaning of the sacrificial fire were the first two boons granted without hesitation. The third boon was to explain the meaning of death, which even Yama, the Lord of Death finds difficult to explain. Refusing to take alternative offers of worldly blessings, Nachiketa's persistent questioning about death and immortality forms the philosophical discourse in Katha Upanishad.  

The superiority of good (shreyas) over pleasant (preyas) and the realization through insight rather than reason are discussed. The concept of body as a chariot carrying the self (Atman) also takes shape in this Upanishad. There are some passages common to the Bhagavad-Gita and the Katha Upanishad. 

Prashna Upanishad 

Prashna Upanishad belongs to the Atharva Veda. Six questions are asked of sage Pippalada. His answers form a dissertation on the question of creation, human personality and metaphysical principle in man. A series six questions such as, ' From where came the creatures of birth?' ' Who is the greatest God?' 'How many Gods are there and how many are manifest? How does life enter the body?' 'What are consciousness, sleep and state of being awake? What is it that knows enjoyment?' 'Which heaven does a man attain who meditates on 'Aum'?' and 'What are the divisions of Purusha?' are answered in this Upanishad. The person bestowed with all the sensory perception of touch, smell, hearing, taste, movements and excretion along with the mind (manas), intellect (buddhi), egoism (ahankara) and the conscious-self (vijnanatman) should take refuge in the supreme imperishable Self (Atman). 

Prajapati, the creator was desirous of offspring (praja) in this world. After performing austerity, he produced a pair of matter (rayi-feminine gender) and life (prana- masculine gender), thinking, 'These two will form creatures for me in many ways'. Prashna Upanishad is thought to be from a later period than the others. 

Mundaka Upanishad 

Belonging to the Atharva Veda, Mundaka literally means 'shaven.' It is the most poetic of the Upanishads. It proposes the way of life as an ascetic (sanyasin). The name is derived from the root mund, 'to shave,' as he that comprehends the teachings of the Upanishad is liberated form error and ignorance, signified by shaving of the head. It also recognizes the superiority of knowledge over performance of sacrifice and actions. It clearly distinguishes between higher knowledge of the Supreme Brahman and the lower knowledge of the empirical world. Only the sanyasin who has renounced everything can obtain the highest knowledge. 

Mandukya Upanishad 

Named for the sage-teacher Mandukya, this Upanishad belongs to the Atharva Veda and contains twelve verses. It is famous for the theory of the four states of mind of humans. These are waking, dreaming, profound sleep and the fourth stage calledturiya, which alone is real. The relationship of these four states to the mystical syllable 'Aum' (or 'Om') is explained. This Upanishad has had a great influence in the subsequent Indian thought.  

The Self - Brahman has four stages or four fourths. The first fourth is the state of waking, when one is outwardly aware, enjoying all that is seen around him. This is the 'Common-to-all-men fourth.' The second fourth is the state of dreaming, when one is inwardly aware and is called 'Brilliant.' The third fourths is the state of profound sleep, when one who is asleep has no desires and dreams. It is a state of 'enjoying bliss and state of thoughtfulness and cognition.' The principal of aum as consisting of three elements a, u, m, which refer to the first three states are explained in this Upanishad. The last fourth is the one that should be discerned. It is the state of tranquility, benignity and without comparison. It is the state of 'being one with the Self.' Mandukya Upanishad alone is said to the enough to lead one to liberation. It was extensively commented upon by both Shankara and his teacher Gaudapada. 

Taittiriya Upanishad 

This Upanishad is a part of the Taittariya Aranyaka and belongs to the Yajur Veda. In it the teacher is describing the ethical rules and the proper dharma to his pupils. It is also famous for the 'Five Sheaths' of the self, namely food, breath, mind, intellect and bliss. It analyzes the five features of the self - prana, vyana, apana, udana and samana. It has three sections called Vallis. The first called Siksha Valli is the first of the six Vedangas (auxiliaries of the Veda): it is the science of phonetics and pronunciation. The second called Brahmananda Valli and the third Bhrgu Valli deal with the knowledge of the Supreme Self, paramatma-jnana. 

Aitareya Upanishad 

Part of Aitareya Aranyaka belonging to the Rig Veda, this Upanishad deals with creation and life after death in more clear terms. It is also famous for its doctrine of Atman as intellect. Intelligence that is Brahman and the Self is the guidance of all creation. All living creatures and the five elements, namely, earth, wind, space, water, light- are the creation of intelligence or the Brahman. 

Aitareya Upanishad mentions three classes of men who aspire to acquire wisdom. The highest class consists of those who have turned away form the world and whose minds are free. The second group intends to attain knowledge more gradually by the worship of prana, the life breath. The third class of men cares for worldly possessions and for them the meditative worship of the Samhita is intended. 

Chandogya Upanishad 

The name is derived from 'chandoga', the name of certain priests specializing in the Sama Veda, the singers of the Saman. It is one of the oldest and best-known Upanishads. A well-known passage is the conversation between Satyakama Jabala and his mother, leading to the assertion that the status of the caste Brahmin is attained by character than by birth. Aum, as the mystical symbol of loud (udgitha) chanting (saman) of Rig (hymns) Veda is explained. The identity of the Self and the Brahman as one and the same as taught by philosopher Aruni is contained in this Upanishad. This doctrine is expressed in the famous saying 'Tat tvam asi' (That art thou). The interesting theory of creation pertaining to the cosmic-egg theory is also delineated here. 

Brhadaranyaka Upanishad 

The oldest and recognized as the most important of all the Upanishads, Brhadaranyaka derives its name from 'great forest-book'. It contains three khandas or sections. The Madhu Khanda expounds the teachings of the basic identity of the individual and the Universal Self. The greatest of Upanishadic philosophers, sage Yajnavalkya's dialogues with his wife, Maitreyi are featured in the Muni Khanda (or Yajnavalkya Khanda). The third is the Khila Khanda which deals with certain modes of worship and meditation.  

It is in this Upanishad the famous doctrine of 'Neti, Neti' ('not this, not this') appears, suggesting the indescribability of the Brahman, the Absolute. It is possible to describe Brahman as what It is not but not as to what It is. Yajnavalkaya makes an effort to define Brahman. The three cardinal virtues of self-restraint (DAmana), alms giving (DAna) and compassion (DAya) are also mentioned here (the three 'DA's). 

Shvetashvatara Upanishad 

Named after the sage, this is one of the later Upanishads. It belongs to the Yajur Veda. There is emphasis here, for the first time, on theism rather than Absolutism that is seen in other Upanishads. The dualism of Sankhya philosophy that appeared later, namely purusha and prakriti are overcome, but many terms used in the Shvetashvatara Upanishad do occur in the Sankhya literature. Advaita philosophy (that later Shankara referred to), with the idea of maya (illusion) made by Ishvara, the manifested God finds a more clear expression in this doctrine. The emphasis in this Upanishad is not on Brahman the Absolute but on the personal Ishvara, the omniscient and omnipotent manifested form of Brahman. 

Kaushitaki Upanishad 

This Upanishad is a part of the Brahmanas of the Rig Veda. Prana, the breathing spirit as the prime mover of the universe is taught by sage Kaushitaki. Prana is also identified with intelligential-self (prajnatman), which is ageless, immortal and bliss. Shankara refers to it in several places in his commentaries on the Brahma-sutra. 

Maitri Upanishad  

Also known as Matrayaniya Upanishad, it belongs to the Black Yajur Veda. Sage Maitri describes two kind of Atman. The 'phenomenal' or the 'bhutatman' reaping the fruits of good and bad karma is the changing self. The 'noumenal' Atman is theAtman that is unaffected by the phenomenal world and abides 'in its own greatness'. This cannot be perceived by the senses but only can be intuited by the intellect. The illusory character of the world and the transience of the phenomenal world implied to here suggest Buddhist thought and influence. 

Maitri Upanishad is of much later date than the classical Upanishads. Mention is made here of the Trimurthi concept of Brahma, Vishnu and Shiva. The three forms are traced to the three gunas, namely rajas, sattva and tamas.