New observations with atmospheric neutrinos from the underground experiments SuperKamiokande, Soudan 2, and MACRO, together with earlier results from Kamiokande and IMB, are reviewed. The most recent observations reconfirm aspects of atmospheric flavor content and of zenith angle distributions which appear anomalous in the context of null oscillations. The anomalous trends, exhibited with high statistics in both sub-GeV and multi-GeV data of the SuperKamiokande water Cherenkov experiment, occur also in event samples recorded by the tracking calorimeters. The data are well-described by disappearence of nu_mu flavor neutrinos arising in oscillations with dominant two-state mixing, for which there exists a parameter region allowed by all experiments. In a new analysis by SuperKamiokande, nu_mu -> nu_tau is favored over nu_mu -> nu_s as the dominant oscillation based upon absence of oscillation suppression from matter effects at high energies. The possibility for sub-dominant nu_mu -> nu_e oscillations in atmospheric neutrinos which arises with three-flavor mixing, is reviewed, and intriguing possibilities for amplification of this oscillation by terrestrial matter-induced resonances are discussed. Developments and future measurements which will enhance our knowledge of the atmospheric neutrino fluxes are briefly noted.
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