This book is an attempt at an English translation of Patanjali's Yogasutra with commentary rendered in current psychological idiom. It features an extensive Introduction to the context and attempts to draw out conclusions on the implications of yoga theory and practices to current psychological knowledge. Yoga Philosophy of Patanjali is based on his Yoga Sutras. In the Yoga Sutras Patanjali brought together various principles and practices of Yoga prevalent at his time into a coherent system. The sutras are short forms with a few essential words each. The Yoga Sutras were composed by man named Patanjali. However, there is not much known about him, except that he was presumably Indian and lived somewhere between the second and fourth century BC. Patanjali is also credited with writing the Mahabhasya, a treatise of Sanskrit grammar and a commentary on Charaka Samhita, the basic text of Ayurveda. Whether they are the same or different people remains a scholastic argument. This book thus provides the psychological context and the relevance of studies of yoga for advancing the existing psychological knowledge. Yoga psychology provides the foundation for Indian psychology, an emerging discipline, rooted in classical Indian tradition.