The doctrine of impermanence can be called the most salient feature of the Buddha’s teaching. The early Buddhist doctrine of impermanence can be understood in four different but interrelated contexts: Buddha’s empiricism, the notion of conditioned/constituted objects, the idea of dependent arising, and the practical context of suffering and emancipation. While asserting the impermanence of all phenomena, the Buddha was silent on the questions of the so-called transcendent entities and truths. Moreover, though the Buddha described Nibbāṇa/Nirvāṇa as a ‘deathless state’ (‘amataṃ padam’), it does not imply eternality in a metaphysical sense. Whereas the early Buddhist approach to impermanence can be called ‘phenomenal’, the post-Buddhist approach was concerned with naumena (things in themselves). Hence, Sarvāstivāda (along with Pudgalavāda) is marked by absolutism in the form of the doctrines of substantial continuity, atomism, momentariness, and personalism. The paper also deals with the approaches to impermanence of Dharmakīrti and Nāgārjuna, which can be called naumenal rather than strictly phenomenal. For Dharmakīrti, non-eternality was in fact momentariness and it was not a matter of experience but derivable conceptually or analytically from the concept of real. Nāgārjuna stood not for impermanence, but emptiness (śūnyatā), the concept which transcended both impermanence and permanence, substantiality and non-substantiality.