Neutrinos are fundamental particles, which interact only weakly with other matter, and had been thought to be massless. However, if they did have some non-zero mass, they would change flavors as they fly along.  

This talk follows the search for these "oscillations" in neutrinos coming from cosmic-ray interactions with the Earth's upper atmosphere using the Super-Kamiokande experiment, and the MINOS experiment, which is making precise measurements of the phenomenon by creating an intense, well-understood neutrino beam at Fermilab (near Chicago) and observing it 735km away at the Soudan Mine in Northeast Minnesota.  

Lastly, the implications of a whole lot of slightly massive neutrinos sloshing around the universe are discussed.