Jupiter's moon Io is one of the most remarkable and dynamic objects in the solar system. Extremely high heat flow, produced by tidal heating, generates extensive volcanic activity. This leads to a unique surface geology and chemistry, a dynamic though tenuous atmosphere, and a complex interaction with Jupiter's magnetosphere, which generates extensive clouds of ionized and neutral material that fill the magnetosphere. Our understanding of Io has increased greatly in the last few years, thanks to intensive study from ground-based telescopes, Earth-orbiting telescopes including the Hubble Space Telescope (HST), and particularly because of the close-up observations by the Jupiter-orbiting Galileo spacecraft. Galileo flew 900 km above Io in December 1995 and has since been observing Io periodically from distances as close as 300,000 km.