Abstract This article examines Edgar Allan Poe’s engagements with nineteenth-century thermodynamic theory via his broader literary explorations of a principle of cosmic deterioration. Focusing especially on the untimely apparitions of entropy and universal heat death in “The Fall of the House of Usher” (1839) and “Mask of the Red Death” (1842), it argues that these apparitions derive from Poe’s creative responses to two primary sources. The first is Epicurean atomism, the most important exposition of which Poe found in Lucretius’s De rerum natura, as mediated by the agonistic interpretations of English natural philosophers Erasmus Darwin (1731–1802) and John Mason Good (1764–1827). The second is the energetic conception historian of science Stephen Brush calls “the wave theory of heat,” which Poe absorbed from contemporary experimental natural philosophers and popularizers of science, including Dionysius Lardner (1793–1859) and John W. Draper (1811–1882). These sources enabled Poe to conceptualize the universe as a system in which irreversible change occurs due to inevitable loss in the transmission or transformation of energy. Poe gave proleptic, poetic expression to this concept in his writings, leading to their haunting echoes in later formulations of, and responses to, entropy and universal heat death.