KamLAND ( Kamioka Liquid-scintillator Anti-Neutrino Detector) is a 1,000 ton active neutrino detector located in a mine a kilometer under the peak of a mountain near Mozumi, Japan. KamLAND has observed anti-neutrinos coming from the world's nuclear reactors for the last three years. The observed neutrino spectrum confirms the evidence from solar neutrino experiments that solar neutrino puzzle is explained by matter-enhanced neutrino-neutrino oscillations. KamLAND also provides the most direct evidence so far for neutrino-oscillation. Recently, KamLAND has observed anti-neutrinos coming from the earth itself, demonstrating that neutrino detectors can be a powerful new tool for geophysical studies of the earth's interior.