  The standard blazar paradigm consists of a supermassive black hole that
expels relativistic jets of magnetized plasma in our direction. This plasma
entrains nonthermal synchrotron-emitting electrons that furthermore scatter
internally-produced synchrotron photons as well as externally-produced photons
from the accretion-disk, the broad-line region, and the infrared-emitting
torus. This picture has been very successful in reproducing the two-humped
blazar spectral energy distribution. Yet various discontents persist, including
(1) ultra-short variability at TeV energies that is much shorter than the black
hole's dynamical timescale; (2) very-high energy (VHE; >100 GeV) radiation from
FSRQs; (3) evidence for a hard spectral component in high-synchrotron peaked
objects, found when deabsorbing the measured VHE spectrum using conventional
extragalactic background light (EBL) models; (4) an unusual slowly varying
class of BL Lac objects; (5) location of the gamma-ray emission region. Some of
these problems can be resolved by introducing a hadronic component into the
blazar paradigm, consistent with the hypothesis that blazars are sources of
ultra-high energy cosmic rays. Gamma-ray observations with the Cherenkov
Telescope Array and neutrino observations with IceCube will be important to
test this hypothesis.
