Classic psychedelic substances, such as lysergic acid diethylamide and the active compound in magic mushrooms, psilocybin, are being studied again in a renaissance of psychedelic research. Psychedelic substances have profound effects on perception, emotion, and cognition, as well as the capacity to induce mystical-type experiences and ego-dissolution. Recent clinical studies indicate that these substances have positive effects on patient populations and healthy participants, both acutely and long-term. Neuroimaging studies show that psychedelics alter neural integration, by the disintegration of normally stable resting state networks, and increasing network connectivity between normally anticorrelated networks. This thesis will review the phenomenological characteristics of the psychedelic-induced altered state of consciousness, the therapeutic potential of the psychedelic-induced altered state of consciousness, and neuroimaging studies on the psychedelic state. Two theoretical accounts are compared on the brain basis of psychedelic-induced altered state of consciousness. From the recent research on psychedelics a novel theory of conscious states has evolved, the entropic brain theory. This theory will be compared to the integrated information theory, a well-established theory of consciousness within cognitive neuroscience.