In chapter 12 we discussed the theoretical framework to describe the CMB brightness (or temperature) anisotropies. These temperature fluctuations have been firmly established and provide very tight constraints on the cosmological parameters. Attention has then should be focused now on the properties of the polarized component of the CMB anisotropy. In fact, scalar modes produce a polarisation signal which is about 10% of the temperature anisotropy, the so-called E-mode. This polarisation mode has been accurately measured by ground-based [112], sub-orbital [150] and orbital experiments [19][7]. Furthermore, tensor modes are expected to produce a polarisation signal on large angular scales with a very specific signature, the so-called B-mode. There is a wealth of scientific information encoded in the CMB polarized signal, which will yield a detailed picture of the recombination and reheating of the intergalactic medium along with the discovery of a primordial stochastic gravitational-wave background. The goal of this chapter is to discuss the basic properties of the E- and B-modes of the CMB polarisation pattern and its dependence on the cosmological parameters.