The essence of Yoga consists of a vast editorial project whose objective is to gather all the material of articles, lectures, interviews, question and answer sessions and training courses offered by Yogacarya Iyengar. This second volume contains an exposition of the eight members of yoga (astanga yoga), a detailed presentation of hatha yoga and, in its third part, the explanation given by the author of his specific contribution to yoga. Thus, this volume constitutes the nucleus of the theoretical building of what is called the "Iyengar Yoga". The work, which has been selected, corrected and personally supervised by B.K.S. Iyengar, is destined to become another reference point for both yoga practitioners and those who wish to know and deepen this incomparable spiritual tradition.