The Korea Microlensing Telescope Network (KMTNet) is a wide-eld photometric system installed by the Korea Astronomy and Space Science Institute (KASI). Here, we present the overall technical specications of the KMTNet observation system, test observation results, data transfer and image processing procedure, and nally, the KMTNet science programs. The system consists of three 1.6 m wide-eld optical telescopes equipped with mosaic CCD cameras of 18k by 18k pixels. Each telescope provides a 2.0 by 2.0 square degree eld of view. We have nished installing all three telescopes and cameras sequentially at the Cerro-Tololo Inter-American Observatory (CTIO) in Chile, the South African Astronomical Observatory (SAAO) in South Africa, and the Siding Spring Observatory (SSO) in Australia. This network of telescopes, which is spread over three dierent continents at a similar latitude of about 􀀀30 degrees, enables 24-hour continuous monitoring of targets observable in the Southern Hemisphere. The test observations showed good image quality that meets the seeing requirement of less than 1.0 arcsec in I-band. All of the observation data are transferred to the KMTNet data center at KASI via the international network communication and are processed with the KMTNet data pipeline. The primary scientic goal of the KMTNet is to discover numerous extrasolar planets toward the Galactic bulge by using the gravitational microlensing technique, especially earth-mass planets in the habitable zone. During the non-bulge season, the system is used for wide-eld photometric survey science on supernovae, asteroids, and external galaxies.