The treatise by Wilhelm Schickard (1592–1635) must be counted among the most significant interventions on the comets of 1618. This is because of its high scientific quality, its length, and the wide variety of aspects it touches on, and not least because of the beauty of the abundant color illustrations with which Schickard illustrated his treatise. Schickard’s work, however, has remained unpublished to this day, even if a digital reproduction is available through the Württembergische Landesbibliothek in Stuttgart. Although Schickard’s Cometen Beschreibung In zwen underschidliche Partes abgetheilt has received some attention, it has not been given all the attention that it doubtless deserves. It is our aim to consider the genesis of the treatise, the advanced cometary theory it proposes (connected with the adoption of heliocentrism and the motion of the Earth) and the observational and theoretical explanation it offers for the three comets of 1618, especially the third and largest, in the context of the history of comets since 1475, as well as Schickard’s pessimistic conjectures regarding the ‘meaning’ of the comet, that is, its effects.