One of India's great yogi's and sage's - Sri. Swami Sivananda (1887 - 1963)
was a medical doctor before he decided to renounce the worldly path and devote his life to the service of humanity and the study of Vedanta. Vedanta is that philosophy which comes from the sacred scriptures called The Upanishads. The Upanishads are the final part of the ancient texts known as the Vedas. He published a wealth of books (over 300 books, pamphlets and journals) on the teachings of yoga. His inspired writings convey the most complex philosophical subjects in a most straightforward and uplifting manner. He established an Ashram and Yoga Academy in Rishikesh, and founded the Divine Life Society in 1935,dedicated to the ideals of truth, purity, non-violence and self-realisation.
yoga disciplines science of yoga 
was one of the many exceptional disciples of Sri Swami Sivananda. Swami Vishnu-devananda came to San Francisco in 1957, on the instruction of Swami Sivananda, to spread the practice of Yoga in the west. After several years of lecturing and demonstrating asanas throughout the United States, he established the international network of Sivananda Yoga Vedanta Centres and Ashrams. After observing the patterns of living in the West, he synthesized the wisdom of yoga into five basic principles proper relaxation, proper exercise, proper breathing, proper diet, positive thinking and meditation which westerners could incorporate into their lifestyle to provide a foundation for the experience of health and peace. Swami Vishnu-devananda was dedicated to the cause of world peace.
"I have given you all I have with all my heart and love. It is up to you to give to others." - Swami Vishnudevananda.
"Then you'll realise your joy, your happiness, your lightness come not from external things, not from goods, not from what you posess, not from what you buy, but from something that comes from within yourself. This is the beauty of meditation. The need for external things drops away because you have become independent of external things. They drop away. And as I said, joy and lightness come from within yourself."
- Godwin Samararatne