Theoretical implications of the CMB spectral distortions

The spectral distortions of the Cosmic Microwave Background (CMB) are quite informative on the processes that might (or had to) occur in the primordial plasma after the thermalization time, during an epoch which is crucial for the formation of the observed structures of the universe. Here we review the relevant processes generating CMB spectral distortions. We compare the presently available data on the spectrum with the predictions of a variety of theoretical scenarios. We also review the connections among the spectral shape of the CMB, the dipole anisotropy and the Sunyaev-Zeldovich effect.