Atmospheric neutrinos: Muon and electron neutrinos problem

This talk discusses the low ratio of muons to electrons in neutrino interactions detected in large underground detectors. This ratio, lower by 4 sigma from the theoretical predictions, has led to interpretations in terms of important phyisical processes such as neutrino oscillations. The emphasis of the talk is on the experimental and theoretical advances during the last two years since the TAUP'93 workshop and on their interpretation in terms of the physical process that causes the abnormal result.