Polarization of the Microwave Background: theoretical framework

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.