Concepts in Hevajra Tantra are not too clearly defined in the text. Theoretical notions and practical guidelines lie scattered throughout the text. In order to get a better comprehension of the same, it is essential to categorize and delve deeper into the various principles and themes of Hevajra Tantra. There is to be found a clear network of ideas existing within the work once the various concepts have been systematically set apart, expounded and studied. An analysis of the text reveals that the basic theoretical foundations and premise of the text are Mahayanic. Hevajra Tantra itself is a work written in the later states of development of Mahayana Buddhism, more precisely Vajrayana school of Buddhism. Thus the entire premise itself is based on Mahayanic principles and understanding, with a more refined outlook. The core concepts of Madhyamika and Yogacara lie scattered throughout the text. These have been collectively referred to as the `Mahayana Foundations.

The prime concepts underlying the theory and practice of Hevajra Tantra are identified below.
Absolute as Tathata
Situated at the core of the philosophical foundations of tantra is the Madhyamika institution of Tathata. It asserts the fundamental unity of nirvana and samsara, of mystical and sensual experiences, and it regards all means as relative to the needs of the practiser. The entire process is kept in check by the conservative tendencies of the Yogacaras, and therefore, the whole movement remains essentially Buddhist. According to the Madhyamika, the Real is one, uniform, undifferentiated nature. It is transcendent to thought as non-relative, non-determinate, non-discursive, non-dual and quiescent. It is referred to in many ways- as tathata, bhutakoti, dharmata, dharmadhatu and sunyata.

Tathata is the ultimate experience in which the real and the intuition of it are non-different (advaya). Tathata does not believe in differentiation and degrees, nor is it the result of the accumulation of Knowledge. Tathata is achieved through the purification of the intellect, which in turn arouses the intuition (prajna).

The Vajrayanic equivalent of tathata is vajra. Bhagavan equates sunyata with Vajra in the Yogaratnamala when he says that the void, sunyata, which is the firm essence- indestructible, indepletable, indivisible, and which could not be consumed- is called vajra.

The same Madhyamika insight is also found reverberated in the Hevajra Tantra. The first verse in chapter five says that the Real is devoid of constructive imagination (sarvakalpanak-sayarupam) and it is non-dual (advaya). It states that there is no duality in the real (grahya-grahakabhava). The Hevajra Tantra is in perfect agreement with the Yogacara view as well, that the absolute is a non-dual consciousness and that the subject-object duality does not pertain to it. "By their very nature, there is neither form nor the one who sees neither sound nor listener, neither smell nor the one who smells neither taste nor the one who tastes neither touch nor the one who touches and there is neither mind nor thought." It is believed that in terms of the True Principle, tattva, smell, sound, form, mind etc. do not exist. Everything such as, the six senses, the Aggregate of the Five Components of Phenomenal Awareness, mantra, deity, is an aspect of the undifferentiated nature, or `thusness` (tathata). It is due to ignorance that things, which are essentially non-existent, appear as though they actually exist outside one`s consciousness. Due to ignorance again, all things appear to be in bondage, but in truth all things are `released`.

These passages of the text echo the ultimate identity of nirvana and samsara as taught by the Madhyamika. The text upholds the unique Vajrayanic insight that the Real is Innate (sahaja) and is of the nature of Great Bliss (mahasukha).

Absolute as Tathagata
Tathata and Tathagata are concerned as two intertwined entities. If the former is considered as prajna or sunyata, the latter may be considered as characterised by the principles of both sunyata and karuna (compassion). Tathatd is the impersonal Absolute, while Tathagata is the principle of mediation between the absolute and the phenomenal. All absolute systems require a mediator, and that need is fulfilled by Isvara in Vedanta philosophy and the Tathagata in Madhyamika and Yogacara vijnanavada. It is believed that only someone who enjoys a sort of dual status, with one foot in the phenomena and the other in the absolute can possibly know the truth and reveal it to others. This serves as the basis for the concept of Tathagata which is complicated metaphysically, but a theological necessity nonetheless. This dual nature of the tathagata, as one with the Absolute and yet actively pursuing the welfare of all beings, serves as the philosophical basis for the theological concept of the trikaya of the Lord Buddha, namely the dharmakaya, the sambhogakaya and the nirmanakaya.

This duality of the Madhyamika institution of Tathagata is found re-iterated in the Hevajra Tantra. It states that, "The Auspicious One enters into the Thusness (tathata) and similarly returns (agata). By this kind of wise reasoning he is known as Tathagatha."

In the text, Tathagata is called Bhagavan, Vajradhara, or the Auspicious One. Bhagavan refers to Vajradhara in the form of Hevajra. He is endowed with bhaga, that is, the six qualities of lordliness, abundance, grace, excellence, splendour and meaningful application of knowledge. The text adopts the three kayas as expounded by the Mahayana tradition and associates them with the various chakras of the tantric subtle body. In addition to the three kayas envisaged by Mahayana, Vajrayana has incorporated a fourth kaya called the mahasukhakaya (the body of Bliss). The fourth kaya is the fusion of the above three kayas, which is the state of the Tathagata (Hevajra) and it is realised through the complex Hevajra sadhana.

Concept of Tathagatagarbha
At the root of all tantric sadhana lies the mahayanic concept of tathatgatagarbha. All beings contain within themselves the seed of enlightenment and are potential Tathagatas (tathagatagarbha). It has been stated in the Madhyamikakarika that nirvana is not something that can be achieved through abandonment or achievement. It is in fact the eternal status of all sentient beings. Mahayana believes that all beings are already Buddhas. This conviction is reaffirmed by the Hevajra Tantra. It states that, "There exists no one being who is unenlightened from the awakening to his own nature. By their very intrinsic nature, the beings in hell, ghosts, animals, gods, titans, men and even worms and so on in the dung, are eternally blissful, for they do not merely experience the pleasure of the gods and the titans". However, the sentient beings do not experience the enlightened state due to the jneyavaranas and klesavaranas. Therefore, the goal of Hevajra Tantra is not to produce enlightenment (bodhi). It is intended to enable the sadhaka to realise that he is essentially of the nature of bliss as he is already enlightened.

Concept of Avidya
Avidya, according to Mahayana, is the prime reason that prevents man from realising his enlightened nature. For the Madhaymika, avidya leads to constructive imagination (kalpana or vikalpa). The main causative factor of Samsara is engaging in views (drsti) and holding on to them. This leads to attachment and aversion, all of which leads to suffering. Avidya for the Yogacarin is the objectification of consciousness (visaya drsti) and the creation of subject-object duality (grahya-grahaka). "The Absolute consciousness is non-dual, but when infected by the illusory idea of the `other`, it is diversified into the subject and object. The function of avidya is the creation of this fundamental duality."

In the Hevajra Tantra are found terms like ajnanam and aguntakamala in order to refer to the obscurations that hide the enlightened nature of things. It is said in the hevajra tantra, "Truly all beings are enlightened beings but they are veiled by the accumulation of defilements. By removing this veil of defilements, all beings are enlightened beings without doubt". These defilements or avidya are said to consist of two avarnas- jneyavarana and klesavarana. Jneyavarana includes such drstis or vikalpas as thoughts of worthiness and unworthiness (bhagabhagavicara), likes and dislikes (istanistvikalpa) etc. It has been said that men are deluded by not knowing the real (loko muhyati vetti na tattvam). Bhagavan in the text adopts the yogacara view that defilements are due to the existence of subject-object duality (grahyagrahakabhavat). Incorrect and wrong views generate afflictions (klesavarna). It is by the purification of these defilements that thusness is realised and attained.

Concept of Bodhichitta
Bodhichittotpada is the name given to the disciple who is undertaken to attain bodhi (Enlightenment). The principal features of the bodhichitta are sunyata and karuna. Sunyata is prajna, intellectual intuition, and is identical with the absolute. The active principle of compassion is the karuna. It gives concrete expression to the sunyata in phenomena. Thus bodhicitta is a unique blend of intellect and will; it becomes the foundation of all altruism.

The Mahayanic tradition preaches the realisation of bodhi through the practice of the Six Paramitas and the Ten Bhumis. In Vajrayana, bodhichitta is seen as comprising two aspects- the absolute and the relative. From the aspect of the absolute, it is understood to be the supreme mystical experience, and as such may be called mahasukha, which is self-experienced (svasariwedya) and sahaja (innate). As relative, it is equated with life-force, the essence of samsara. This again has two forms- the female coefficient and the male coefficient.

There are a total of three nadis in the tantric conception of the subtle body, namely the lalana, rasana and avadhuti. In the tantric conception of the subtle body, there are three important nadis, namely lalana, rasana and avadhuti. At the base of the generative organ where lalana and rasana unite and avadhuti ascends, bodhichitta exists in its relative form (sukra). At the crown of the head bodhicitta exists in its absolute condition as mahasukha, known also as the Moon. Prajna or Sunyata is the female and the male is the upaya or the karuna. Bodhichitta in its relative aspect is achieved through the union of the yogi with the yogini. This limited experience strengthens the resolve of the practitioner and gives him a foretaste of the ultimate enlightened consciousness. It is believed that yogic practices enable the sadhaka to retain the relative bodhichitta at the nirmana cakra and force it upward through the avadhuti to ultimately unite with the bodhichitta residing at the usnisakamala or mahasukha cakra, the highest cakra. This union is spoken of as the union of the lady with the lord and is of the nature of great bliss. This union can also be achieved through a process of inner union of the two nadis in meditation according to the guru`s instructions.

Nature of Nirvana
Nirvana is considered as something almost inexpressible by the different schools of Buddhism. It is not something that can be physically attained. The transformation that takes place is not so much ontological (objective) as it is epistemic (subjective). So what actually undergoes a transformation in the entire process is the attitude of the seeker. According to the Madhyamika, the nature of nirvana is indeterminate (asamskrta). The Yogacarin considers the state of nirvana to be the state of consciousness which is rid of subject-object duality and rests in itself. This nirvana is of the nature of supreme bliss (mahasukha).

Nirvana is not a separate reality or a different state from that of saimdra. It is said that the absolute is the only real. It is nothing but phenomena (samsara) viewed without the distorting medium of constructive imagination.

A number of different terms have been employed by the hevajra tantra in order to signify nirvana- siddhi, sahaja, mahasukha, mahamudra etc. For the Vajrayanist, the accomplishment of the Mahamudra is without distinction (adbhedyalaksana) and unachieved (asiddha). It is, as also viewed by the Yogacarin, where consciousness rests in itself and does not create. It is a state of supreme delight. Throughout the Hevajra Tantra, this aspect of the bliss of nirvana has been highlighted.

Thus discussed are the primary concepts of Hevraja Tantra. Most of the theory and praxis of the tantra flows forth and takes shape from the same.