An atomic-scale quantum conductance switch is demonstrated which allows to open and close an electrical circuit by the controlled and reproducible reconfiguration of individual silver atoms within an atomicscale junction [1,2]. The only movable parts of the switch are the contacting atoms. The device, which is fabricated by electrochemical deposition of silver atoms, is entirely controlled by an external voltage applied to an independent third gate electrode. Controlled switching was performed between a quantized, electrically conducting “on-state” exhibiting a conductance of G0 = 2e2/h (≈ 1/12.9kΩ) or preselectable multiples of this value and an insulating “off-state”. The device, which reproducibly operates at room temperature, represents an atomic transistor or relay, opening intriguing perspectives for the emerging fields of quantum electronics and logics on the atomic scale.figure