We are developing techniques for ultrafast sampling of THz waveforms by using femtosecond laser pulses. This “THz oscilloscope” can monitor electron motions in nanostructures with 0.1 ps-time resolution. By using this technique, we are investigating dynamical carrier transport in quantum nanostructures and exploring possibilities of realizing novel ultrahigh-frequency oscillators, such as superlattice Bloch oscillators.