Experimental(1-7) and theoretical(8,9) investigations have demonstrated that a quasi-two-dimensional electron gas (q-2DEG) can form at the interface between two insulators: non-polar SrTiO(3) and polar LaTiO(3) (ref. 2), LaAlO(3) (refs 3-5), KTaO(3) (ref. 7) or LaVO(3) (ref. 6). Electronically, the situation is analogous to the q-2DEGs formed in semiconductor heterostructures by modulation doping. LaAlO(3)/SrTiO(3) heterostructures have recently been shown(10) to exhibit a hysteretic electric-field-induced metal-insulator quantum phase transition for LaAlO(3) thicknesses of 3 unit cells. Here, we report the creation and erasure of nanoscale conducting regions at the interface between two insulating oxides, LaAlO(3) and SrTiO(3). Using voltages applied by a conducting atomic force microscope (AFM) probe, the buried LaAlO(3)/SrTiO(3) interface is locally and reversibly switched between insulating and conducting states. Persistent field effects are observed using the AFM probe as a gate. Patterning of conducting lines with widths of similar to 3 nm, as well as arrays of conducting islands with densities >10(14) inch(-2), is demonstrated. The patterned structures are stable for >24 h at room temperature.