  We present a brief review of the polarization properties of the cosmic
microwave background in dark matter models for structure formation. Quite
independently of the model parameters, the polarization level is expected to be
$\sim 10%$ of the anisotropy signal at angular scales $\le 1^o$. Detections of
polarization at larger angular scales would provide a strong evidence in favour
of an early reionization of the intergalactic medium.
