These ten are the most important and principal texts. Modern scholars believe that these also represent the oldest of the upanishadic texts. Others add the kaushItakI and SvetASvatara upanishads to the list of principal upanishads, and some add the maitrAyaNI too.

Other upanishads: A number of other upanishads are extant today. The Indian traditions regard the upanishads as Sruti, which is timeless, eternal, and apaurusheya (unauthored). Hence, finding dates of composition for the various upanishads is a meaningless non-issue to them. Some of the texts that are called upanishads may not be accepted by specific traditions. However, this is really a question of acknowledging the Sruti status of a given text, not one of finding a date of composition. Modern scholars attempt to fix periods of composition for all these texts. Needless to say, the date of composition of these texts, including those of the principal upanishads, is of no real concern to the vedAnta traditions.

It has become popular to classify the upanishads in terms of the subject matter they cover. Thus, we have a large number of upanishads dealing with general topics of vedAnta, in addition to those that teach yoga, and those that detail the rules of sam.nyAsa. The upanishads that tend to concentrate on one of the Great Deities of Hinduism are usually classified as Saiva, vaishNava and SAkta upanishads.

A list of 108 upanishads as found in the muktikopanishad is given in the table below. The yajurveda column has two rows under each heading - kRshNa yajurveda texts are in the upper row, and Sukla yajurveda texts in the lower one.

Comments: The classification of the upanishads on the basis of their subject matter seems reasonable, and other than the 10 principal ones, most of the upanishads quoted by the earliest commentators fall under the sAmAnya vedAnta category. However, some upanishads could possibly be classified under more than one heading. For example, varAha and pASupatabrahma upanishads are classified as yoga upanishads and not as vaishNava and Saiva upanishads respectively. Similarly, gaNapati upanishad is included as a Saiva upanishad, while skanda upanishad is not. Also, hamsa upanishad is called a yoga upanishad and not a sam.nyAsa upanishad, whereas paramahamsa is included as a sam.nyAsa upanishad. Similarly, the mahAvAkya upanishad and the brahmavidyA upanishad might also justifiably be included under the sam.nyAsa upanishads.

In any case, there seems to be a large overlap in subject matter between the "yoga" upanishads and the "sam.nyAsa" upanishads, pointing to the close relationship between yoga practice and sam.nyAsa as an institution. This also raises the possibility that the traditional association of yoga with sAm.khya in terms of the six darSanas may be slightly misleading. In this connection, it is interesting to note that the most important texts on the yoga system are by teachers of advaita vedAnta, from Sankara downwards, although all these commentators explain yoga more or less in sAm.khyan terms. Another interesting observation in this connection is that advaita vedAntins have completely internalized yoga practice as an aid to meditation and to realize the non-dual brahman.

The bulk of the vaishNava (9 out of 14), Saiva (6 out of 14) and SAkta (5 out of 9) upanishads are assigned to the atharva veda. However, it should be noted that the other three vedas also have a significant share of the "late" upanishad texts. Three SAkta upanishads are from the Rg veda, while there are no vaishNava upanishads and only one Saiva upanishad assigned to the Rg veda. Also, there are no Saiva or SAkta upanishads assigned to the Sukla yajurveda, but a substantial number of Saiva (5 out of 14) upanishads are from the kRshNa yajus. The SAkta upanishads are grouped together, although some teach worship of sarasvatI, lakshmI or pArvatI, and others describe SrIcakra upAsanA, where Sakti is identified with brahman Itself, rather than being the Sakti of one of brahmA, vishNu or Siva. Since the upanishads are associated with individual SAkhAs within each veda, it might be interesting to investigate the distribution of these upanishads further, and correlate them with the distribution of the vaidika SAkhAs among today's communities of vaishNavas, Saivas and SAktas.