  Relativistic shocks are one of the most plausible sites of the emission of
strongly variable, polarized multi-wavelength emission from relativistic jet
sources such as blazars, via diffusive shock acceleration (DSA) of relativistic
particles. This paper summarizes recent results on a self-consistent coupling
of diffusive shock acceleration and radiation transfer in blazar jets. We
demonstrate that the observed spectral energy distributions (SEDs) of blazars
strongly constrain the nature of hydromagnetic turbulence responsible for
pitch-angle scattering by requiring a strongly energy-dependent pitch-angle
mean free path. The prominent soft X-ray excess ("Big Blue Bump") in the SED of
the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton
scattering of external radiation fields by the thermal electron population,
which places additional constraints on the level of hydromagnetic turbulence.
It has further been demonstrated that internal shocks propagating in a jet
pervaded by a helical magnetic field naturally produce polarization-angle
swings by 180$^o$, in tandem with multi-wavelength flaring activity, without
requiring any helical motion paths or other asymmetric jet structures. The
specific application of this model to 3C279 presents the first consistent,
simultaneous modeling of snap-shot SEDs, multi-wavelength light curves and
time-dependent polarization signatures of a blazar during a polarization-angle
(PA) rotation. This model has recently been generalized to a lepto-hadronic
model, in which the high-energy emission is dominated by proton synchrotron
radiation. It is shown that in this case, the high-energy (X-ray and
$\gamma$-ray) polarization signatures are expected to be significantly more
stable (not showing PA rotations) than the low-energy (electron-synchrotron)
signatures.
