In 1997, anticipating Galileo’s arrival in the Jovian system, predictions were made about the petrology of Io’s interior and lavas. Voyager observations had discovered sulfurous volcanism, but it was Earth-based telescopic measurements that indicated that at least some lavas were above the boiling point of sulfur. Io’s tidally-driven heat flow is so great that the volume of lava erupted over the age of the Solar System could have been as much as 40 times the volume of Io. Given that magmas are generated by partial melting, this suggests that each part of Io has undergone igneous differentiation hundreds of times and Io should be extraordinarily differentiated with a highly felsic crust and an extremely depleted dunitic mantle. Erupting lavas were predicted to have melting temperatures <1100 °C, producing a ~50-km thick feldspar-rich crust similar to the lunar highlands. Mountains were hypothesized to be formed by large buoyant plutons. In 1998 Galileo found lava eruption temperatures of 1500-1700 °C, requiring that at least one assumption in this model for Io is fundamentally wrong. Unlikely options are that (a) Io’s bulk composition is not chondritic or (b) tidal heating of Io is a very recent phenomenon. Alternatively, Io’s crust is 100% recycled into the mantle, short-circuiting differentiation. This would be most plausible if the base of the crust had such a high degree of partial melting that it mechanically disaggregated into a magma “mush” on a global scale. Between 1999 and 2007, a self-consistent model for Io was developed with a mushy magma ocean trapped under a crustal lithosphere with extreme compression. While no real alternative to this mushy magma ocean hypothesis was advanced, the idea was not widely accepted. However, the 2011 re-analysis of the Galileo magnetometer data suggested a global high conductivity layer that is consistent with such a magma ocean. The idea remains controversial but has engendered great interest because understanding how Io’s interior is tidally heated is essential for determining the history of Europa’s potentially habitable ocean. It also could provide new insight into how Enceladus can be active, the early history of the Earth and Moon, and the geology of some exoplanets. The Discovery-class Io Volcano Observer will write the next chapters of this story.