The single-atom transistor, which was first demonstrated by scientists in Karlsruhe in 2004, allows the opening and closing of an electrical circuit by the controlled and reproducible reconfiguration of an individual atom within an atomic-scale junction. The only movable part of the switch is the contacting atom. The device is entirely controlled by an external voltage applied to a third, independent gate electrode. Controlled switching is performed between a quantized, electrically conducting “on-state”, exhibiting a conductance of G0 = 2e2/h (≈ 1/12.9 kΩ), 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 logic on the atomic scale.