Five years after Voyager 1 and 2 encountered Jupiter's intriguing satellite Io, many geologic questions remain open. What is the composition of Io? How large a part does sulfur play in the formation of landforms? Are the flows seen on Io sulfur or silicate, and how are they emplaced? What methods can we use to find out? Finally, what can the upcoming Galileo mission to Jupiter tell us that will help answer these questions?