  Astrophysical and atmospheric neutrinos are important probes of the powerful
accelerators that produce cosmic-rays with EeV energies. Understanding these
accelerators is a key goal of neutrino observatories, along with searches for
neutrinos from supernovae, from dark matter annihilation, and other
astrophysics topics. Here, we discuss how neutrino observatories like IceCube
and future facilities like KM3NeT and IceCube-Gen2 can study the properties of
high-energy (above 1 TeV) neutrino interactions. This is far higher than is
accessible at man-made accelerators, where the highest energy neutrino beam
reached only 500 GeV. In contrast, neutrino observatories have observed events
with energies above 5 PeV - 10,000 times higher in energy - and future large
observatories may probe neutrinos with energies up to $10^{20}$ eV. These data
have implications for both Standard Model measurements, such as of low
Bjorken$-x$ parton distributions and gluon shadowing, and also for searches for
beyond Standard Model physics. This chapter will review the existing techniques
and results, and discuss future prospects.
