  The $\gamma$-ray emission observed in several classes of Galactic and
extragalactic astrophysical sources appears to be linked to accreting black
holes and rotational powered neutron stars. These systems are prodigious cosmic
accelerators, and are also potential sources of the UHE cosmic rays detected by
several experiments and VHE neutrinos. We review a recent progress in our
understanding of these objects, and demonstrate how recent and future
observations can be employed to probe the conditions in the sources.
