The Galileo mission provided an unprecedented opportunity to observe Io's volcanic activity at close range. The Near-Infrared Mapping Spectrometer (NIMS) mapped the thermal emission from Io's many volcanic regions, as well as the distribution of sulfur dioxide and of a yet-unidentified compound that absorbs near 1 micron. Repeated fly-bys of Io by Galileo revealed for the first time the eruptive styles at different hot spots, which range from lava lakes to long, insulated lava flows similar to those found in Hawaii. Observations at global scales have provided insight into the distribution of hot spots over the surface, which can lead to constraints on interior models. This talk will summarize the NIMS results to date, and outline the major questions that will need to be addressed by future observations from the ground and from space.