Breakthrough observations by EGRET of high-energy gamma-ray blazars, pulsars, unidentified sources, delayed emission from gamma-ray bursts and solar flares, and diffuse emission from our Galaxy and beyond, have all changed our view of the high-energy Universe and raised many new questions. A major new mission called the Gamma-ray Large Area Space Telescope (GLAST) is now being planned to make tremendous steps forward. The mission is being led in the US by NASA and the DOE with major contributions from Europe and Japan. GLAST will provide enormously improved sensitivity, angular resolution and high-energy >100 GeV response compared to EGRET. The typical sensitivity of 2x10^-9 ph cm^-2 s^-1 for E>100 MeV will lead to detections of 1000's of sources. Source localizations of arcmin size will allow counterpart identification for most sources. The mission is approved for launch in 2005.