The appearance of high energy tau neutrinos due to nu_mu-nu_tau
oscillations of extragalactic neutrinos can be observed by measuring
the neutrino induced upward hadronic and electromagnetic showers and
upward muons. The tau neutrino regeneration in the earth, and the
implications for extracting oscillation signals for a variety of
extragalatic flux models will be described, as well as issues of high
energy tau propagation in materials such as water or rock.