Atmospheric neutrinos are produced by cosmic ray interactions with the air nuclei in the atmosphere. In 1998, neutrino oscillations were discovered by the studies of these neutrinos with the Super-Kamiokande experiment, a water Cherenkov detector with the total mass of 50 kilo- tons. In this lecture, I will discuss the experimental studies that led to the discovery of neutrino oscillations, which showed that neutrinos have very small mass. The implications of the small neutrino masses, as well as the status and the future prospect of neutrino experiments, will also be discussed briefly.