Abstract: An electro mechanical bicycle derailleur actuator system, retrofittable to a bicycle having gearing and at least one derailleur, the derailleur having a cable, the system comprising: at least one derailleur actuator module (DAM) connectable to the bicycle and to the cable; a cyclist interface module (CIM) connectable to the bicycle for cyclist interface with the system; and a control and power module (CPM) connectable to the bicycle serving to control and power the system, wherein the bicycle gearing is shiftable by the system without derailleur cable modification.