ATMOSPHERIC NEUTRINOS - MUON AND ELECTRON NEUTRINOS PROBLEM

This talk discusses the low ratio of muons to electrons in neutrino in teractions detected in large underground detectors. This ratio, lower by 4 sigma from the theoretical predictions, has led to interpretation s in terms of important phyisical processes such as neutrino oscillati ons. The emphasis of the talk is on the experimental and theoretical a dvances during the last two years since the TAUP'93 workshop and on th eir interpretation in terms of the physical process that causes the ab normal result.