A history of the exploration of Io -- A summary of the Galileo mission and its observations of Io -- Formation and early evolution of Io -- The interior of Io -- Ionian mountains and tectonics: Insights into what lies beneath Io's lofty peaks -- Active volcanism: Effusive eruptions -- Plumes and their deposits -- Io's surface composition -- Io's atmosphere -- Io's neutral clouds, plasma torus, magnetospheric interaction -- Outstanding questions and future explorations. Jupiter's moon Io is the Solar System's most exotic satellite. Active volcanism on Io was discovered from observations by the Voyager 1 spacecraft in 1979, confirming a possibility suggested from theoretical studies of Io's orbit. Our knowledge of Io's volcanism, composition, and space environment were significantly increased as a result of observations by the Galileo spacecraft from 1996 through 2001. The end of the Galileo mission in 2003 makes this an ideal time to summarize the new results in a book as no book has ever been written about Jupiter's volcanic moon, Io.