This book explores one of the most explicit and sophisticated theoretical formulations of tantric yoga. It explains Abhinavagupta's teaching about the nature of ultimate reality, about the methods for experiencing this ultimate reality, and about the nature of the state of realization, a condition of embodied enlightenment. The author uncovers the conceptual matrix surrounding the practices of the Kaula lineage of Kashmir Shaivism. The primary textual basis for the book is provided by Abhinavagupta's Parātrīśikā-laghuvṛtti, a short meditation manual that centers on the symbolism of the Heart-mantra, SAUḤ.