In this paper, the authors show an original methodology for optimisation of tool geometry used in the milling of spherical surfaces. This methodology is based on thorough theoretical research, which highlights the kinematic characteristics of spherical surface generation by milling. These characteristics lead to the conclusion that the constructive angles of tools used in spherical surface milling must have the same values on both cutting edges. In this paper, a practical design methodology of these tools is also presented, both for exterior spherical milling, and for interior spherical milling.