The book describes the major physical theories of comets in the past two millennia. It demonstrates the evolution of ideas about the nature, position, motion and physical constitution of comets from Aristotle to Whipple. Unlike the available works on the history of comets, which either illustrate relatively short periods in the history of physical cometology or portray a landscape view without adequate details, the present study focuses on details of each theory. It also investigates the interaction between observational and mathematical astronomy, and the physical sciences in defining the properties of comets. Ideas about the astrological aspects of comets are not considered in this study. Also, topics concerning the motion of comets are explained to the extent that is helpful in illustrating their physical properties. Although the present study is mainly focused on the physical theories of comets, its results will be relevant to studies in the history of geology, planetary science, and astrology. On the other hand, those results may initiate new studies about educational practices for physics and astronomy in post-Newtonian Europe, the ways that different parts of Newton's physical, astronomical and cosmological ideas evolved after him. Also, the debates about the constitution and chemical properties of comets in the post-Laplacian era may trigger new researches about possible influence of cometary studies on the foundation of astrophysics.