Throughout history, people have described alterations of consciousness in which they experienced a sense of transcendence, being part of a much larger unity, sometimes encompassing the whole universe, or of being pure awareness. These experiences have been foundational for many religious and spiritual practices, albeit they have sometimes been treated as suspect or outright psycho- and or neuro-pathological. This chapter defines various aspects of the self, self-transcendence, and mystical experiences (ME), discusses the relation of ME with other constructs, its prevalence, psychological and demographic correlates, and its relation to mental health, reviews brain-imaging research on it, and concludes with a discussion of its ontological implications. By doing so, it seeks to give an overview of both enlightenment experiences, ‘the light,’ and the underlying neurophysiological processes, ‘the bulb’.