  A certain degree of linear polarization has been measured in several GRB
afterglows. More surprisingly, circular polarization has been recently measured
in GRB121024A. For synchrotron emission, the polarization level depends on: (i)
the local magnetic field orientation (ii) the geometry of the emitting region
with respect to the line of sight and (iii) the electron pitch-angle
distribution. For this reason, polarization measurements are a valuable tool to
probe afterglow micro-physics. We present numerical estimates of linear and
circular polarization for different configurations (i.e., magnetic fields,
geometries and pitch-angle distributions). For each different scenario, we
study the conditions for reaching the maximum and minimum linear and circular
polarization and provide their values. We discuss the implication of our
results to the micro-physics of GRB afterglows in view of recent polarization
measurements.
