  The standard inflationary model presents a simple scenario within which the
homogeneity, isotropy and flatness of the universe appear as natural outcomes
and, in addition, fluctuations in the energy density are originated during the
inflationary phase. These seminal density fluctuations give rise to
fluctuations in the temperature of the Cosmic Microwave Background (CMB) at the
decoupling surface. Afterward, the CMB photons propagate almost freely, with
slight gravitational interactions with the evolving gravitational field present
in the large scale structure (LSS) of the matter distribution and a low
scattering rate with free electrons after the universe becomes reionized. These
secondary effects slightly change the shape of the intensity and polarization
angular power spectra (APS) of the radiation. The APS contain very valuable
information on the parameters characterizing the background model of the
universe and those parametrising the power spectra of both matter density
perturbations and gravitational waves. In the last few years data from
sensitive experiments have allowed a good determination of the shape of the
APS, providing for the first time a model of the universe very close to
spatially flat. In particular the WMAP first year data, together with other CMB
data at higher resolution and other cosmological data sets, have made possible
to determine the cosmological parameters with a precision of a few percent. The
most striking aspect of the derived model of the universe is the unknown nature
of most of its energy contents. This and other open problems in cosmology
represent exciting challenges for the CMB community. The future ESA Planck
mission will undoubtely shed some light on these remaining questions.
