Abstract: Trochoidal tooth form gear assemblies for in-line mechanical power transmission, gear reduction and differential drive employ internally meshed gear sets having differing numbers of trochoidal gear teeth, all of which are in continuous contact as one gear rotates and orbits relative to another. Coupling and output rotors having external trochoidal gear teeth profiles rotate and/or orbit in mesh with conjugate internal trochoidal gear teeth in stators or coupling rotors. A differing number of teeth between the meshed external and internal gear teeth provides gear reduction of rotational speeds through axially aligned input and output shafts. In certain embodiments, couplers are eccentrically mounted upon axially extending pins to translate the orbital motion of a meshed gear to shaft rotation. The gear sets can be combined in series to provide various gear reduction ratios as functions of the differing number of meshed gear teeth.