KamLAND (Kamioka Liquid-scintillator Anti-Neutrino Detector) is a 1,000 ton active neutrino detector located in a mine one kilometer under a mountain peak near Mozumi, Japan. KamLAND has observed anti-neutrinos coming from the world’s nuclear reactors for the last four 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. It has been used in searches for unexpected anti-neutrinos from the sun and it provided evidence of anti-neutrinos coming from the earth, demonstrating that neutrino detectors can be a powerful new tool for geophysical studies of the earth’s interior.