  With the discovery of evidence for neutrino mass, a vivid gamma ray sky at
multi-TeV energies, and cosmic ray particles with unexpectedly high energies,
astroparticle physics currently runs through an era of rapid progress and
moving frontiers. The non-vanishing neutrino mass establishes one smooth
component of dark matter which does not, however, supply a critical mass to the
Universe. Other dark matter particles are likely to be very massive and should
produce high-energy gamma rays, neutrinos, and protons in annihilations or
decays. The search for exotic relics with new gamma ray telescopes, extensive
air shower arrays, and underwater/-ice neutrino telescopes is a fascinating
challenge, but requires to understand the astrophysical background radiations
at high energies. Among the high-energy sources in the Universe, radio-loud
active galactic nuclei seem to be the most powerful accounting for at least a
sizable fraction of the extragalactic gamma ray flux. They could also supply
the bulk of the observed cosmic rays at ultrahigh energies and produce
interesting event rates in neutrino telescopes aiming at the kubic kilometer
scale such as AMANDA and ANTARES. It is proposed that the extragalactic
neutrino beam can be used to search for tau lepton appearance thus allowing for
a proof of the neutrino oscillation hypothesis. Furthermore, a new method for
probing the era of star formation at high redshifts using gamma rays is
presented which requires new-generation gamma ray telescopes operating in the
10-100 GeV regime such as MAGIC and GLAST.
