Michael Maestlin (1550–1631), professor of mathematics at the University of Tübingen, was a leading protagonist of the astronomical and cosmological revolution that began with Copernicus. Famous for first introducing Copernicanism to Kepler, Maestlin also wrote important treatises on the supernova of 1572 and the comet of 1577 that mark significant steps in the elimination of celestial immutability and the reinforcement of the Copernican worldview. This first critical edition of Maestlin’s German manuscript treatise on the comet of 1618 is accompanied by an English translation and a thorough commentary. An extensive introduction situates Maestlin’s treatise in the broader context of the contemporary politico-religious conflict and cosmological discussion newly expanded to the debate on sunspots discovered with the telescope.