• In ancient China, portentology was a “science” in its own right, a specialised field of knowledge developed by rational individuals who endeavoured to fathom the concealed mechanisms at work beneath the spectacles of history and the world at large. This paper focuses on the nomenclature of portents (observed phenomena interpreted as auspicious or inauspicious signs) displayed in the chapters of the Han-era official historiography known as the “monographs” (or “treatises”) on celestial phenomena and the Five Agents, examining the pre- and early imperial sources of this typology and its underlying classificatory principles, and assessing how this epistemic object has been dealt with by modern scholarship.