During the last decades of the 20th century, several experiments measuring the Cosmic Microwave Background (CMB) anisotropy spectrum (for example COBE, TOCO, BOOMeranG, MAXIMA, DASI, CBI) contributed significantly to our understanding of the properties of our Universe. The next step forward will be the measurement of the CMB polarization, whose very weak signal represents a real challenge to experimentalists. Moreover, non-negligible contamination...