  With the discovery of a high-energy neutrino flux in the 0.1 PeV to PeV range
from beyond the Earth's atmosphere with the IceCube detector, neutrino
astronomy has achieved a major breakthrough in the exploration of the
high-energy universe. One of the main goals is the identification and
investigation of the still mysterious sources of the cosmic rays which are
observed at Earth with energies up to several $10^5$ PeV. In addition to being
smoking-gun evidence for the presence of cosmic rays in a specific object,
neutrinos escape even dense environments and can reach us from distant places
in the universe, thereby providing us with a unique tool to explore cosmic
accelerators. This article summarizes our knowledge about the observed
astrophysical neutrino flux and current status of the search for individual
cosmic neutrino sources. At the end, it gives an overview of plans for future
neutrino telescope projects.
