Mario Mateo observes the stars and motion of objects in nearby galaxies to assess the distribution of mass, particularly dark mass, within them. He has designed and built fiber spectrographs for Magellan that can analyze an unprecedented number of targets – more than 250 – at a time. His new Michigan/Magellan Fiber System (M2FS) is the most powerful yet and is ideal for studying objects’ chemical composition, mass, temperature, and motion. Mateo has recently completed an complementary spectroscopic capability, IFUM, that will uniquely among comparable instruments be able to carry out detailed spectroscopic studies simultaneously over entire astronomical objects such as dwarf galaxies, star-forming regions, star clusters and active galactic nuclei. These tools will allow UM and Magellan observers to carry out a wide range of key spectroscopic studies in many areas of study.