  The WMAP first year detection of a high redshift reionization through its
imprints on CMB anisotropy T and TE mode angular power spectra calls for a
better comprehension of the universe ionization and thermal history after the
standard recombination. Different reionization mechanisms predict different
signatures in the CMB, both in temperature and polarization anisotropies and in
spectral distortions. The Planck capability to distinguish among different
scenarios through its sensitivity to T, TE, and E mode angular power spectra is
discussed. Perspectives open by future high sensitivity experiments on the CMB
polarization anisotropy and spectrum are also presented.
