  The matter content of relativistic jets in AGNs is dominated by a mixture of
protons, electrons, and positrons. During dissipative events these particles
tap a significant portion of the internal and/or kinetic energy of the jet and
convert it into electromagnetic radiation. While leptons - even those with only
mildly relativistic energies - can radiate efficiently, protons need to be
accelerated up to energies exceeding $10^{16-19}$ eV to dissipate radiatively a
significant amount of energy via either trigerring pair cascades or direct
synchrotron emission. Here I review various constraints imposed on the role of
hadronic non-adiabatic cooling processes in shaping the high energy spectra of
blazars. It will be argued that protons, despite being efficiently accelerated
and presumably playing a crucial role in jet dynamics and dissipation of the
jet kinetic energy to the internal energy of electrons and positrons, are more
likely to remain radiatively passive in AGN jets.
