The contents of the paper cover: a brief history of some concepts of the theory of gearing, and new developments in this area, (i) theory of enveloping curves and surfaces, (ii) simulation of meshing and contact of gear tooth surfaces, (iii) basic kinematic relations as the key to the determination of direct relations between the curvatures of contacting surfaces, and avoidance of singularities, (iv) transfer from line contact of tooth surfaces to point contact, and (v) design and manufacturing of gears with compensated transmission errors caused by misalignment.