This book examines and clarifies the nature, meaning, significance and vitality of the sacred (and the profane), in relation to some of the diverse religions of the world and the rich and multifarious traditions of the sacred in many cultures and times, in the context of ontology (broadly, the philosophical study or investigation of being). It provides incisive critical analyses and evaluations of many important contributions to our understanding of the sacred, and the holy, especially in relation to the world's religions, religious experience, religious insight or knowledge, metaphysics, mythology and mysticism. A number of important theories and explanations are also critically analyzed and evaluated, including the numinous theory of the sacred and the holy (Otto), the psychodynamic theory (Freud), the sociological theory (Durkheim), empirical theories (Russell and Ayer), the ontological question (Heidegger) and the hierophantic theory (Eliade)-among others. The book concludes with a number of reflections on the ontology of the sacred (and the profane) in relation to philosophy and science, that will open up new pathways of thinking, reflection and investigation in the 21st century.