Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration [1,2]. We demonstrate the reversible control of the insulator-metal transition (IMT) in a two dimensional (2D) electron gas at the interface of insulating SrTiO3 single crystals. Superconductivity was observed in a limited number of devices doped far beyond the IMT, which may imply the presence of 2D metal-superconductor transition. This realization of a two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.The perovskite SrTiO 3 is sometimes called the "silicon of oxide electronics" because it is commonly used as a substrate for epitaxial growth of a variety of oxide films [3]. Not only it is an insulator with a band gap of 3.2 eV but also a quantum paraelectric with an extremely large dielectric constant of more than 10 6 at low temperatures [4]. N-type conduction has been realized by introducing a small number of oxygen vacancies or by cation substitutions. An insulator to metal transition occurs at a rather low carrier concentration of n ∼1018 cm −3 [5, 6] for a doped oxide system, which owes largely to the high mobility exceeding 10 4 cm 2 /Vs. Superconductivity emerges in a limited concentration range from n = 7 × 10 18 to 5 × 10 20 cm −3 and the transition temperature T c shows a bell shaped dependence on n with an optimum T c ∼ 0.35 K at n ∼ 10 20 cm −3 [7]. The carrier concentration required to achieve superconductivity is two orders of magnitude smaller than that for high-T c cuprates which is a great advantage to realize electrostatic control of insulator-metal and superconductor transitions.Such a unique doping properties of SrTiO 3 has triggered attempts to dope charged carriers electrostatically by using a field effect transistor (FET) structure. However, so far, attempts to realize metallic state have been unsuccessful [8]. This is due to several reasons, notably carrier trapping at the gate insulator/SrTiO 3 interface, Schottky barrier formation between SrTiO 3 and source and drain electrodes, and insufficient breakdown voltage of the gate insulators. An alternative approach has shown that two dimensional electron gas can be created at SrTiO 3 /LaAlO 3 hetero-interface by a charge-transfer due to a polar discontinuity [9,10,11,12,13,14] or possible oxygen defects [15]. This electronically tailored interface was found to be superconducting as in the bulk material [14].Recently the performance of SrTiO 3 FET has been drastically improved [16,17] by overcoming the contacts and interface problems. These new devices showed metallic behavior down to 7 K [17]. In this Letter, we extend our study to lower temperatures below 1 K, and show that the two-dimensional insulator-metal transition is indeed achieved at the interface between parylene and SrTiO 3 reversibly while sweeping the gate voltage. A superconducting transition to zero-resistance state was obs...