  GRB afterglow polarization is discussed. We find an observable, up to 10%,
polarization, if the magnetic field coherence length grows at about the speed
of light after the field is generated at the shock front. Detection of a
polarized afterglow would show that collisionless ultrarelativistic shocks can
generate strong large scale magnetic fields and confirm the synchrotron
afterglow model. Non-detection, at a 1% level, would imply that either the
synchrotron emission model is incorrect, or that strong magnetic fields, after
they are generated in the shock, somehow manage to stay un-dissipated at
``microscopic'', skin depth, scales. Analytic lightcurves of synchrotron
emission from an ultrarelativistic self-similar blast wave are obtained for an
arbitrary electron distribution function, taking into account the effects of
synchrotron cooling. The peak synchrotron flux and the flux at frequencies much
smaller than the peak frequency are insensitive to the details of the electron
distribution function; hence their observational determination would provide
strong constraints on blast wave parameters.
