The search for circuitry that is not bound by the limits of conventional technologies has lead to research on a variety of radically different approaches to future electronics. The basic idea behind this work is to exploit physical effects in structures that are ultra small (nanometer size) in one or more dimensions. Quantum effects, such as resonant tunneling are being used in heterostructures with ultra-small vertical dimensions while ballistic electron motion, electron-wave interference, single-electron tunneling, and other effects are being explored in nanostructures with ultra-small lateral dimensions such as quantum point contacts, quantum dots, and quantum wires. In summary, a wide range of work aimed at developing future electronic circuitry based on physical effects in devices with ultra-small dimensions is being carried out in Japan. Functional operation of resonant-tunneling digital circuits has been demonstrated.