Bhakti Yoga in Vaishnavism mainly refers to Bhakti as an upaya. It is believed that the sole purpose of Brahman knowledge is to attain Brahman, as says the Taittiriya Upanishad. The entire purpose of all the philosophic knowledge that has been acquired is that it should culminate in the attainment of the highest spiritual goal. This important teaching of the Upanishad pre-supposes a practical discipline or means (upaya) which has to be undertaken in order to attain the goal. According to the Vishishtadvaita Vedanta, this upaya is either Bhakti Yoga or Prapatti Yoga that serves as the direct means to achieve the supreme human goal (Parama Purusartha). Both are important means for Moksha and have been advocated as alternative methods intended for two different categories of individuals having different capacities and conditions of eligibility. The Upanishads and the Vedanta-sutra have given greater emphasis to Bhakti Yoga, also known as Upasana, whereas the Vaishnava treatises have accorded prominence to Prapatti.

Etymology of Bhakti in Vaishnavism
The term Bhakti is derived from the root word `bhaj` which means seva or meditation (bhaj sevayam). In common usage it is understood in the sense of love towards the respected or elderly person (mahaniyavisaye pritih). Priti or love is a state of knowledge, a mental disposition. Bhakti with reference to God, therefore, means unceasing meditation with intense love for the Supreme Being (snehapurvam anudhyanani).

The general concept of Bhakti in the sense of devotion or reverence to a deity is accepted by all religions. However, this kind of Bhakti of a general character is not regarded as the direct means to Moksha by the Vishishtadvaita Vedanta. Here, Bhakti as a Sadhana for Moksha refers to the rigorous religio-spiritual discipline to be undertaken by a qualified aspirant for Moksha, and it is to be pursued continuously for the entire life-time until the total liberation from bondage is secured.

Concept of Bhakti in Upanishads
The term Bhakti by itself is not mentioned in the Upanishads. Where it has been mentioned in the texts, it is referred to be certain other words, such as jnana, vedana, darsana, dhyana, dhruva smriti, nididhyasana and Upasana. The Upanishads also state explicitly that Jnana or knowledge of Brahman is the only means to Moksha. According to Ramanuja, there are various different terms used in the Upanishads such as upasana, dhyana, smriti santati, vedana and darsana which can be taken to mean the same as bhakti referred to in the Bhagavad Gita.

Theory of Bhakti Yoga
The theory of Bhakti Yoga attributes three essential features to it. These are summed up in the Bhagavad Gita in three points.

Firstly, the aspirant is required to fix his mind on the Paramatman with deep devotion to Him (Dhyana).

Secondly, he should engage himself in the worship of the Supreme Being (Yajna)

. Thirdly, he should do other acts such as offering salutations to Him (Namaskara).

The scope of the Bhakti Yoga is far wider than that of the Ashtanga Yoga, which is a part of it. The Vedanta sutra refers to the need of a steady posture, concentration of mind and proper congenial atmosphere for contemplation. The sevenfold ethical discipline, known as Sadhana Saptaka, helps the individual practising Bhakti Yoga to achieve good progress in the meditation on Brahman.

Stages of Bhakti Yoga
The concept of the different stages of Bhakti yoga seem to have been taken from the hymns of Nammalvar and the Bhagavad Gita. Ramanuja, who is the foremost exponent of Bhakti Yoga as a direct means to Moksha, has conceived of three important stages. These are termed as Para Bhakti, Para Jnana and Parama Bhakti. When the unceasing meditation on God is perfected to the extent of its becoming similar to a clear vision, it is regarded as Para Bhakti, or higher form of Bhakti, to distinguish it from the ordinary Bhakti, which is just devotion to God needed in the initial stages of Upasana. The other two terms, Para Jnana and Parama Bhakti represent the next two stages of Bhakti Yoga signifying the intensity of Bhakti of the Upasaka.