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.