Combining data from the recently launched Fermi satellite with TeV observations of atmospheric Cherenkov
telescopes and low-energy observations has improved our understanding of the sources of high-energy radiation as
well as of the conditions in the intergalactic space. After a review of the basic principles of propagation and
detection of high-energy photons, I will discuss three applications: First, I examine for the example of the
nearest radio galaxy, Cen A, how electromagnetic cascades shape the observed gamma spectrum and how they inform
us about the conditions in the source. Second, I show that the non-observation of some TeV blazars in the GeV
range by the Fermi satellite leads to the first lower limit on the intergalactic magnetic field (IGMF). Moreover,
the IGMF has to fill more than ~50% of the Universe, strongly constraining possible generation mechanisms.
Finally, I discuss how the observation of a diffuse extragalactic photon background in the GeV-TeV range can be
used to constrain the flux of high energy neutrinos, providing important information for the choice of future
neutrino experiments.