This English translation, with revisions, of the well-known Chinese edition presents systematically the geometric theory of conjugate tooth surfaces in a more or less rigorous form. The concepts of the two kinds of limit points and limit curves are explained in some detail and a general formula for induced normal curvature is derived, of which the formula of Euler-Savary appears as a direct consequence. The idea of relative differentiation, initiated by Zhida Yan, simplifies the presentation considerably. The phenomenon of secondary contact, closely related to the limit curve of the second...