In both literature and science, descriptions abound of so-called "self-transcendent" experiences, which seem to deeply alter the boundaries between oneself and the rest of the world. The subjects of these experiences often report that they feel extended to the world, united with it, or on the contrary retracted from it, as if they were tiny or insignifcant. Sometimes, they report the disappearance of the very distinction between themselves and the world (ego-dissolution). The aim of the present essay is to give an account of self-transcendent experiences and identify the level of self-consciousness to which they belong. The main claim to be defended is that self-transcendent experiences involve special instances of metacognitive feelings of familiarity or unfamiliarity. Theoretical and empirical research on the latter can then be used to shed light on the former.